@david I think this happens when some point of stress reaches its limit, and due to the nature of the glass, it's basically a chain reaction. If you look at slow motion video of a https://en.wikipedia.org/wiki/Prince_Rupert%27s_Drop shattering you'll get the idea.

What I remember from glassblowing is a lot of annealing to try to let the glass "relax" and not retain internal stresses caused by shaping it. Safety glass like yours is tempered, which distributes stresses as evenly as possible but may, counterintuitively, have incredible internal stresses on purpose. Tempered glass is supposed to shatter like this, rather than leaving big or sharp shards, which as a side effect means it all shatters rather than a crack forming or a piece breaking off. Wherever it's resting on a few points to support weight will become an external stress point and will likely be where the crack starts, although you'll never see it because of the chain reaction. What you saw is inevitably the result.