act of god or random destruction

I got home yesterday to see that the top piece of glass in my screen door was completely smashed. Some of the glass was on the ground and some was still hanging on for dear life. There was no rock, no brick with a nasty note tied to it, no damage to the screen, no blood, nothing that would have given me a clue about why and how it happened. The good news is that when the weather gets nicer I can have the screen door open--before we could never get the glass out of the door because it was stuck.