Extreme stress, aka throwing a very heavy package on top of yours while the bubble mailer is lodged at an angle. I made a quick drawing of how I believe things (normally get bent, but also likely damaged in other ways, like slabs cracking). When a bubble mailer lands in a sorting bin at an angle and is stuck there it is way more susceptible to being crushed or bent because there is nowhere for the weight to brace i.e the package below normally, It's just thin air.
Here is the masterful drawing I made of a postal worker throwing an 80 pound box on top of a 4 oz bubble mailer that's lodged in the bin. Blue line is your item.
