I don't understand what's so difficult to understand.
The phone bends itself.
If we take two conditions that we all agree on: empty aluminum unibody at rest does does not want to flex, and gorilla glass neither, the answer is simple.
The internal hardware that is secured to the aluminum unibody introduces torsional stress into the phone. Over time, this manifests itself into a bend, as the stress of how the internal hardware is affixed to the phone forces it into a preferred state to relieve the torsional pressure.
There is inherent energy in the phone that makes it want to bend. This could be as simple as a single screw point that was affixed using an uncalibrated amount of torque.
Of course if we even take that in its resting state the phone wants to bend, it probably shouldn't. However something so simple as a tap, small drop, hit, etc could momentarily degrades the phones integrity that it will thus begin to bend.
Stop being shills.
The evidence is clear that the phone wants to bend itself to relieve the torsional pressure that exists in the phone.