UPDATE - It seems that the asker was asking about a different explosions (there were two in the story). This covers one, @Keen's answer covers a second one)

This is addressed in the book by Andy Weir that the film is based on (which I highly recommend reading):

TL:DR: it was an explosion resulting from combining the hydrogen with the oxygen that was contained in the air Mark breathed out while sleeping.

(LOG ENTRY Sol 41)
So what happened? Well, I have a theory.
According to the main computer, during the blast, the internal pressure spiked to 1.4 atmospheres, and the temperature rose to 15°C in under a second. But the pressure quickly subsided back to 1 atm. This would make sense if the atmospheric regulator were on, but I’d cut power to it.
The temperature remained at 15°C for some time afterward, so any heat expansion should still have been present. But the pressure dropped down again, so where did that extra pressure go? Raising the temperature and keeping the same number of atoms inside should permanently raise the pressure. But it didn’t.
I quickly realized the answer. The hydrogen (the only available thing to burn) combined with oxygen (hence combustion) and became water. Water is a thousand times as dense as a gas. So the heat added to the pressure, and the transformation of hydrogen and oxygen into water brought it back down again.
The million dollar question is, where the hell did the oxygen come from? The whole plan was to limit oxygen and keep an explosion from happening. And it was working for quite a while before blowing up.
I think I have my answer. And it comes down to me brain-farting. Remember when I decided not to wear a space suit? That decision almost killed me.
The medical O2 tank mixes pure oxygen with surrounding air, then feeds it to you through a mask. The mask stays on your face with a little rubber band that goes around the back of your neck. Not an airtight seal.
I know what you’re thinking. The mask leaked oxygen. But no. I was breathing the oxygen. When I was inhaling, I made a nearly airtight seal with the mask by sucking it to my face. The problem was exhaling. Do you know how much oxygen you absorb out of the air when you take a normal breath? I don’t know either, but it’s not 100 percent. Every time I exhaled, I added more oxygen to the system.
It just didn’t occur to me. But it should have. If your lungs grabbed up all the oxygen, mouth-to-mouth resuscitation wouldn’t work. I’m such a dumb-ass for not thinking of it! And my dumb-assery almost got me killed!

UPDATE - It seems that the asker was asking about a different explosions (there were two in the story). This covers one, @Keen's answer covers a second one)

This is addressed in the book by Andy Weir that the film is based on (which I highly recommend reading):

TL:DR: it was an explosion resulting from combining the hydrogen with the oxygen that was contained in the air Mark breathed out while sleeping.

(LOG ENTRY Sol 41)
So what happened? Well, I have a theory.
According to the main computer, during the blast, the internal pressure spiked to 1.4 atmospheres, and the temperature rose to 15°C in under a second. But the pressure quickly subsided back to 1 atm. This would make sense if the atmospheric regulator were on, but I’d cut power to it.
The temperature remained at 15°C for some time afterward, so any heat expansion should still have been present. But the pressure dropped down again, so where did that extra pressure go? Raising the temperature and keeping the same number of atoms inside should permanently raise the pressure. But it didn’t.
I quickly realized the answer. The hydrogen (the only available thing to burn) combined with oxygen (hence combustion) and became water. Water is a thousand times as dense as a gas. So the heat added to the pressure, and the transformation of hydrogen and oxygen into water brought it back down again.
The million dollar question is, where the hell did the oxygen come from? The whole plan was to limit oxygen and keep an explosion from happening. And it was working for quite a while before blowing up.
I think I have my answer. And it comes down to me brain-farting. Remember when I decided not to wear a space suit? That decision almost killed me.
The medical O2 tank mixes pure oxygen with surrounding air, then feeds it to you through a mask. The mask stays on your face with a little rubber band that goes around the back of your neck. Not an airtight seal.
I know what you’re thinking. The mask leaked oxygen. But no. I was breathing the oxygen. When I was inhaling, I made a nearly airtight seal with the mask by sucking it to my face. The problem was exhaling. Do you know how much oxygen you absorb out of the air when you take a normal breath? I don’t know either, but it’s not 100 percent. Every time I exhaled, I added more oxygen to the system.
It just didn’t occur to me. But it should have. If your lungs grabbed up all the oxygen, mouth-to-mouth resuscitation wouldn’t work. I’m such a dumb-ass for not thinking of it! And my dumb-assery almost got me killed!

This is addressed in the book by Andy Weir that the film is based on (which I highly recommend reading):

TL:DR: it was an explosion resulting from combining the hydrogen with the oxygen that was contained in the air Mark breathed out while sleeping.

(LOG ENTRY Sol 41)
So what happened? Well, I have a theory.
According to the main computer, during the blast, the internal pressure spiked to 1.4 atmospheres, and the temperature rose to 15°C in under a second. But the pressure quickly subsided back to 1 atm. This would make sense if the atmospheric regulator were on, but I’d cut power to it.
The temperature remained at 15°C for some time afterward, so any heat expansion should still have been present. But the pressure dropped down again, so where did that extra pressure go? Raising the temperature and keeping the same number of atoms inside should permanently raise the pressure. But it didn’t.
I quickly realized the answer. The hydrogen (the only available thing to burn) combined with oxygen (hence combustion) and became water. Water is a thousand times as dense as a gas. So the heat added to the pressure, and the transformation of hydrogen and oxygen into water brought it back down again.
The million dollar question is, where the hell did the oxygen come from? The whole plan was to limit oxygen and keep an explosion from happening. And it was working for quite a while before blowing up.
I think I have my answer. And it comes down to me brain-farting. Remember when I decided not to wear a space suit? That decision almost killed me.
The medical O2 tank mixes pure oxygen with surrounding air, then feeds it to you through a mask. The mask stays on your face with a little rubber band that goes around the back of your neck. Not an airtight seal.
I know what you’re thinking. The mask leaked oxygen. But no. I was breathing the oxygen. When I was inhaling, I made a nearly airtight seal with the mask by sucking it to my face. The problem was exhaling. Do you know how much oxygen you absorb out of the air when you take a normal breath? I don’t know either, but it’s not 100 percent. Every time I exhaled, I added more oxygen to the system.
It just didn’t occur to me. But it should have. If your lungs grabbed up all the oxygen, mouth-to-mouth resuscitation wouldn’t work. I’m such a dumb-ass for not thinking of it! And my dumb-assery almost got me killed!

UPDATE - It seems that the asker was asking about a different explosions (there were two in the story). This covers one, @Keen's answer covers a second one)

This is addressed in the book by Andy Weir that the film is based on (which I highly recommend reading):

TL:DR: it was an explosion resulting from combining the hydrogen with the oxygen that was contained in the air Mark breathed out while sleeping.

(LOG ENTRY Sol 41)
So what happened? Well, I have a theory.
According to the main computer, during the blast, the internal pressure spiked to 1.4 atmospheres, and the temperature rose to 15°C in under a second. But the pressure quickly subsided back to 1 atm. This would make sense if the atmospheric regulator were on, but I’d cut power to it.
The temperature remained at 15°C for some time afterward, so any heat expansion should still have been present. But the pressure dropped down again, so where did that extra pressure go? Raising the temperature and keeping the same number of atoms inside should permanently raise the pressure. But it didn’t.
I quickly realized the answer. The hydrogen (the only available thing to burn) combined with oxygen (hence combustion) and became water. Water is a thousand times as dense as a gas. So the heat added to the pressure, and the transformation of hydrogen and oxygen into water brought it back down again.
The million dollar question is, where the hell did the oxygen come from? The whole plan was to limit oxygen and keep an explosion from happening. And it was working for quite a while before blowing up.
I think I have my answer. And it comes down to me brain-farting. Remember when I decided not to wear a space suit? That decision almost killed me.
The medical O2 tank mixes pure oxygen with surrounding air, then feeds it to you through a mask. The mask stays on your face with a little rubber band that goes around the back of your neck. Not an airtight seal.
I know what you’re thinking. The mask leaked oxygen. But no. I was breathing the oxygen. When I was inhaling, I made a nearly airtight seal with the mask by sucking it to my face. The problem was exhaling. Do you know how much oxygen you absorb out of the air when you take a normal breath? I don’t know either, but it’s not 100 percent. Every time I exhaled, I added more oxygen to the system.
It just didn’t occur to me. But it should have. If your lungs grabbed up all the oxygen, mouth-to-mouth resuscitation wouldn’t work. I’m such a dumb-ass for not thinking of it! And my dumb-assery almost got me killed!