About 15 years ago I had a similar thing happen to my '87 Nissan Pathfinder. It had been parked in the driveway overnight, and when I opened the door in the morning I was greeted with an interior full of shattered glass and an open hole in the roof.
At first I thought someone must have broken in overnight, but the doors were still locked, no other windows had been touched and nothing inside had been disturbed. In the end, I figured it had just happened on its own - maybe from a sudden change in temperature and a pre-existing rock chip I hadn't noticed.
Hopefully yours won't cost too much to replace - I'd be very surprised if GM steps up to cover the cost.

happens - especially with glass.