The only logical way I can think of why the iPad would bend like this would be because of a bloated battery. The way these devices are made, there is virtually zero air space within the device. The iPad Air and iPad Air 2 are prime examples of this. Batteries bloat when there's a fault within them. If there's nowhere for the bloated battery to expand to, it will push against whatever is in it's way and either bend it or split it apart. That's a fact and I've had two iPhone 4's batteries bloat, causing most of backglass to lift away from the metal frame surrounding the device. They looked like I had taken a screwdriver to them and tried to pry the back off. Both of the iPhones were replaced as being defective due to bloated batteries.
With the virtually zero air space within today's iPads, what would exactly happen if the battery were to bloat? I would assume the device would split apart...would it bend as yours did? It's possible.
Fact is that Apple didn't even open the device to check for any interior damage such as battery bloating and simply told you that you bent it somehow. I would believe Apple more if they would've opened the device and verified the condition of the battery and if there were any other logical reason why the device would bend.
Good luck trying to get some resolve.
