Probably faulty manufacture. Streess is induced at the suspension points and can start to develop hairline cracks. When one of these cracks reaches a tempering stress cell (the samll pieces of crumb glass you get left with all over the floor) it will propagate explosively through the whole sheet.

Boom and doom in one go.

I've had this happen on tempered glass in a greenhouse roof. (glazing strips were overtightened). Makes a hell of a mess