In The Martian, the "Hab" explodes at some point in the film, specifically the time that the airlock blows out.
Is it explained what exactly happens that causes this?
I watched the film in the U.S. and English isn't my first language, so some details may have been lost for me.
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 isn't explained in the film. In the book, it's explained that a part of the Hab was mildly damaged in the storm that stranded Watney. From the book:
AL102 shuddered in the brutal storm. Withstanding forces far greater than it was designed for, it rippled violently against the airlock seal-strip. Other sections of canvas undulated along their seal-strips together, acting as a single sheet, but AL102 had no such luxury. The airlock barely moved, leaving AL102 to take the full force of the tempest. The layers of plastic, constantly bending, heated the resin from pure friction. The new, more yielding environment allowed the carbon fibers to separate. AL102 stretched. Not much. Only four millimeters. But the carbon fibers, usually 500 microns apart, now had a gap eight times that width in their midst.
AL102 being the Hab component that tore, it was the portion of the Hab connected to the airlock that blew off. Watney kept using the same airlock due to its convenient access to things outside. This meant AL102 kept being stressed until it tore. Again from the book:
AIRLOCK 1 SLOWLY depressurized to 0.006 atmospheres. Watney, wearing an EVA suit, stood inside it waiting for the cycle to complete. He had done it literally hundreds of times. Any apprehension he may have had on Sol 1 was long gone. Now it was merely a boring chore before exiting to the surface. As the depressurization continued, the Hab’s atmosphere compressed the airlock, and AL102 stretched for the last time.
On Sol 119, the Hab breached.
The initial tear was less than one millimeter. The perpendicular carbon fibers should have prevented the rip from growing. But countless abuses had stretched the vertical fibers apart and weakened the horizontal ones beyond use.
The full force of the Hab’s atmosphere rushed through the breach. Within a tenth of a second, the rip was a meter long, running parallel to the seal-strip. It propagated all the way around until it met its starting point. The airlock was no longer attached to the Hab.
