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.

Really? This was one of the first questions I asked our Swedish engineer roommate when he moved in a few years ago. He sort of shrugged and smiled and said: "We don't harbour him any ill-will." Which I suppose left unsaid, "We don't exactly LIKE him, either."