Note that the piece ("bezel") is a piece of plastic separate from the main body of the case. It appears that it is made of some form of graphite or brittle composite (not more forgiving ABS plastic).

It is obvious that this is one of the last pieces pressed into place (pressure fit) in the unit's final assembly. Take note that the headphone jack and the power plugs have their own sleeve.

Immense pressure is required to put, set and keep them in place, so twisting and compression is present.

I assume that these units are hand-assembled, so you have a human error component. I also assume that the fractures are developed from the factory assembly line - when the bezel is pressed into place - most likely because it is OFF CENTERED (effectively becoming a miniature "moment arm" and cracking the piece).

Just my personal analysis.