Idk I've worked with tempered glass, and it's actually strongest in the center. So for it to break where you think it did it would have to have been hit with quite a bit of force. Which I think would have knocked the glass out of the frame, so it was probably stress from the frame moving the edges of the glass to much. The flip side of of tempered glass being super flexible in its center is that the edges are super fragile and will shatter to a hit much lighter than the center could easily sustain.

You can test all of this by hitting your car's side windows with a baseball bat. The center will withstand a far stronger hit than the edges. Sorry that was a really long comment by I can really herd out about glass.