I believe that this question is aimed at an event that happens at the very end of chapter 13. The final moments in this chapter actually switch to the third-person narrative perspective, and that allows the narrator to explain exactly why the Hab failed and launched Watney and the airlock 50 meters away from the Hab.
Readers are told that the airlock depressurized as normal. Like Watney, readers are not expecting anything to go wrong, because Watney has moved himself through this airlock onto the Martian surface hundreds of times. Unfortunately, that repeated usage is the problem. Every time the airlock is pressurized and depressurized, the airlock and Hab undergo stress, and the fibers holding everything together get progressively weaker and weaker. Chapter 13 sees the fibers finally succumbing to the inevitable, and the airlock is fully separated from the Hab in a violent explosion.
Venkat will confirm the weakened canvas hypothesis at the start of chapter 15, and he recommends that Watney now alternate between using airlock 2 and 3 for all EVAs. This is something that in hindsight, Watney should have been doing all along. His repeated usage of airlock 1 was out of convenience, but it caused all of the stress loads to be endured by the canvas by airlock 1. Watney could have minimized a Hab failure by doing nothing more than using a different door each time he needed to exit or reenter the Hab.