1964 Barracuda video Back Glass breaking!

When my Barracuda got delifered from New Mexico, there was a spare rear window(in really nice shape) inside. I took it out, and moved it from the garage to near my basement door, with plans of storing it in my basement. The ground outside my basement door is just 2b limestone, so I set the window on end, and was going to go into the basement and open the door(cannot get it open from the outside), I made sure the glass was secure and wasn't going to fall, and I turned to head into the house, and the rear window just exploded, It didn' start to fall or anything, just exploded. I took me over 2 hrs to get 99.5% of the glass cleaned up out of the gravel. I still find pieces.

FF