There are two types of zipper damage that I see all the time. The pull tabs getting broken off, which happens because they get caught in the belts or in cracks in baggage carts. But usually the zippers are still functional in those instances, just harder to open with nothing to grab onto. The other common one happens because people overpack their bags, to the point where its bulging out, as well as being overweight. I've seen many bags basically explode when put on a belt or in a cart, simply because the stress on the zippers was too much. It also tends to be the cheaper quality luggage that does it more frequently. But nevertheless, that I see as being more the passenger's fault than anything.