How is it possible for a glass shower door to shatter on it's own?

I have a separate shower stall / tub. The partition is a half-wall between the tub and shower tall, and sitting on top of the half-wall is a glass wall. I’m not sure how, but the glass wall spontaneously shattered on it’s own. It started with a loud popping sound, and within 20 minutes the entire glass was shattered.