Taking a (possibly naive) optimistic view, this glass is probably toughened (as opposed to laminated). Given the age of the glass, possible stress within the glass due to temperature / shock etc - it could be that the glass just gave up of it's own accord.
I had this happen to me years ago with an old car with a toughened windscreen. The glass must have had a small stone chip (ie a stress raiser). I parked the car, slammed the door shut, walked up the road - and seconds later, heard a whoose noise as the windscreen collapsed. The slamming action must have caused a small instantanious increase in internal pressure - with the already stressed screen, that was the last straw.
Cobra friends with toughened glass sidescreens and wind wings often complain of them spontaneously exploding. Toughened glass is, already, highly stressed.
I use plexiglass sidescreens and wind wings.