Why Did This Glass Break?

Sunlight?

Something similar happened to a pane of glass left in our backyard. Every day, a shaft of sun would pierce through the neighbouring buildings, falling only on a specific section of the glass. One day, that section just fell out, cleanly. Perhaps the glass was sitting in a windowsill?

In any case, the general consensus here in the comments seems to be that this is some sort of thermal fracture. Day in and day out, the glass was heated and cooled in a specific pattern, stressing the structure of the glass. Then one day, it just fractured, cleanly.

Here's a theory. I'd combine the idea of a warmer glass, and a colder beer, but combine it with the beer being poured in roughly the same spot over and over. If you tip that glass over, as if to pour a beer in, that's the approximate splash area.

I've seen glasses fall apart this way, but I have no idea why they did.

A very small Terminator materialized nearby at just the right moment.