I still maintain that from a technical standpoint, it's perfectly plausible that the glass suddenly and catastrophically failed. Not an explosion per se, just a piece of glass suddenly shattering. It happens, and hardened glass is even more prone to just such a failure. The very process that hardens it also puts it under constant stress, all it would take is a tiny weak spot. I'm not saying this is definitely what happened, but with millions of glass screen covers produced, it's not entirely impossible that a few fail here and there.