Glass can have huge stresses embedded to it if post-heating in fabrication process goes wrong. It's literally a timebomb waiting to go bang.
Another reason to cause unseen hidden stresses is a mistake done in gluing process. In the old days windshields were attached by a slotted rubber band that elastically fixed the glass to the frame. Nowadays the windshield is actually an important part in supporting the body and it's glued directly to the frame. If the gluing goes wrong the glass is left with internal stresses and again bang by surprise. Heat changes can trigger this or if extra stress is added in form of lifting / possibly slightly bending the frame.