OP, terribly sorry to see / hear about your cracked screen. Looking at things objectively, to my eye it looks like something made impact with your phone. It may have not necessarily been a drop but perhaps something dropped onto it.
I've had glass tea mugs explode on me after drinking a hot cup of tea and foolishly running it under cold water. You're talking about a rapid change in temperature which would cause rapid expanding and contracting. That phone would have had to been at a very cool temperature and then quickly heated up in order to shatter.
I'm sorry but I have a hard time believing the heat theory. My car windows get hot and cold with ac / heat. Water application screen protectors recommend using a hair dryer to get the bubbles out. Screen replacements sometimes call for heat guns to loosen adhesive behind the digitizer.
Once again, your phone must have been in the freezer and then thrown in the oven for it to rapidly expand fast enough for that break.
I wish you the best of luck.