Thank you all for your replies! I was surprised how many people have had the same experience. I just never know what the real world is like, ya know? I come here for validation and grounding.
I made an appt to have it replaced, and the girl answering the phone had never heard of a car window just exploding like that! I scared her. But, at work this am, I talked to the guy who manages the fleet of cars for our division, and he's had two cars do that. Once might have been a temperature differential on a very hot day, but he thinks most often it's a rock riccocheting off the car alongside you.
This being a major interstate, there's always big rigs. And we sand and gravel the roads in winter for traction, although most of that has been swept up by now, but there's always rocks on the roads from sand and gravel trucks not covering their loads. And there was a car right alongside me when it happened so it was probably a rock bouncing off him and into me.
Yes we did look for a bullet when cleaning up the glass, but no sign of one. And I'd changed my undies that morning so God can hardly ask for more than that!
And ... no one hurt. No one was sitting in that seat at the time, and I was headed home anyway, and I park in a garage at home. Only $200 to replace it ...
Made it one of those days where there's just too much being dumped on me. An opportunity to practice some recovery - stay calm, don't take things personally, and ask for help when something out of the ordinary happens.