Weirdly cracked window !!

Something happened today and I was wondering if it ever happened to one of you guys..

Me and my friend Pat were in the garage sanding and getting my Cavy done for it's new paint job, and sudently we heard a big "pow" and we saw that the back window suddently cracked completly in one big shot! Like the whole back window cracked in a second!

The window was not hit, or heated.. ..it just decided to crack without any warning! And the worse part is that it's still cracking!

Anybody would have a idea on what could have caused this? A guy told me that every windows on a car have a expiration date, but I find it kinda hard to believe!