>No idea what happened, no clue why it broke.

Probably tweaked it just enough to catch the edge of the glass in the frame.

I work in a window/door factory. I deal with tempered glass all night long. You can blast the center of that glass with the plastic headed hammers we use and nothing happens. Tap that corner though....POP!

I actually had a piece of glass blow up hard enough that the chunks hit and popped a piece of glass behind me.