Believe it or not I have had something similiar happen. While driving down I-40 outside Little Rock, AR in a Dodge Neon, (Sorry that was preToyota Days) Had the drivers side window down approximately 3-4" and all of a sudden it sounded like a gun went off!!! Also lucky there was very little traffic. No overpass to be seen, A truck in front of me had thrown a rock just right and hit the leading edge of the drivers door window. It spider webbed from the point of impact. The whole window was shattered. The only thing that keep the window semi intact was the tint on the window. Scared the Sh!t out of me. Had to carefully roll the window the rest of the way up and it held until I got to Oklahoma City (where I was heading) and just made it to an automotive glass shop, I had called on the way there, right before they closed and got the window replaced.
There is a good chance it was a rock from a truck or one thrown from an overpass. I just think it is unlikely it just blew up on its own.