It is temperature differential (Dan could probably step in better on this than me). The heat differential from one side of the pane of glass to the other means one side is slightly bigger than the other in a solid object, creating a physical stress.

Normally, part of the heat from a defroster goes into a glass, and part of it radiates out. When you cover the defroster elements with thing, you've essentially put a blanket on them. A thin, lousy blanket, but a blanket none the less. If you've gut the window under a little extra stress then you hit a bump that's just hard enough--it shatters.

That's part of why you can touch a defroster element and it doesn't feel hot. It's just a little bit of heat. More would defrost faster--but your window would shatter during the first cold day when you turned it on.

This temperature stress can be going beyond the factory planned limits with heating elements covered in plastic tint, or by black tint on the sunroof getting full sunlight as you pull out of your partially air conditioned garage in Phoenix.