One time when Mark Watney entered the airlock
the plastic-like wall broke. It looked like air started coming in even though the pressure outside should never be higher than the one inside. At the end the whole airlock exploded.
I'm having hard time imagining why this happened. How is this explained scientifically?
@Jack B Nimble's answer gives an excellent explanation of what happens in the book. The thing is that the same exact thing happens in the Ridley Scott's film adaptation.
In the first image we see the internal structure of the Airlock 1. After awhile, we are shown how it starts de-pressurising; however, it is not the airlock itself, but its juncture with the HAB. Just after that, we see the whole airlock structure being completely de-attached and ejected from the HAB.
One has to admit, though, that the scene is definitely fast-paced and a bit misleading, which is why you might have thought that the air was coming from Mars' atmosphere.
From the novel:
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 had 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 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. - Ch. 13: LOG ENTRY: SOL 119
This makes it clear that the internal pressure from the Hab is what blew the airlock and ejected it away from the main structure.
In the book it was the pressure of the Hab itself that violently ejected the airlock, once the hab fabric connector weakened sufficiently (from repeated use) for it fail.
