I had the same thing happen on my convertible in 1971 when the car was only 18 mths old. I drove from Kingston to Mississauga to buy a house and as we were walking away from the car after parking it, the rear window just fell into the back seat and started popping like pop corn. By the time it was finished there wasn't a piece of glass bigger than course salt. Scared the crap out of my poor dog sitting in the middle of the back seat.
The dealer said it was a common tempered glass failure usually caused by stress in the glass sheet and no it wasn't covered by the 5-50,000 mile warranty.
If you cover your window inside and out with clear packing tape you might get it out in one piece. I wouldn't be surprised to find some of those glass bits are still hiding in my car after 48 years.