Why was Warner’s suit malfunctioning in The Martian?

Watney's space suit malfunctions due to problems with the Hab. The Hab simply wasn't designed to be used as much as it is here—it wasn't meant to be used for this many cycles. Over time, Watney's constant venturing in and out of the airlock has placed enormous cyclic stress on the Hab, so much so that it's just a matter of time before the airlock explodes.
When it does, it inevitably damages the EVA suit that Watney's wearing. As well as having to fix the airlock, Watney also has to carry out some running repairs on his space suit; he uses duct tape to fix the airlock and glue to repair his shattered faceplate. When he finally returns to the Hab, Watney's able to get himself another EVA suit and fix the Hab with proper glue and fabric.