Improbable, but not impossible. I've seen it happen.

Also, I actually cracked the windshield in my 88 blazer with 2 18's back in the day. Before the fight starts, I'm sure it was a fluke. My guess is that it wasn't glued in right from the factory, since one corner seemed to move more, and leaked. My theory is that with one loose corner, it flexed funny, and cracked.

But I've seen windows shatter, and windshields blow out. But Like John said, you wouldn't have been able to sit in them when it happened.