How does a car window spontaneously explode?

My dad was driving to work one winter evening when the rear window of this 1997 Chevy Lumina exploded (currently Dec 2010). No shards were left in the frame; it was completely free of glass. His first thought was that someone shot it either with a bb gun or a real gun. But then someone he talked to suggested that the rear defroster may have shorted out causing the explosion. It is still the same night that it happened so my dad has not had a chance to look in his car for any type of projectile. Has anyone heard of this happening?