What would make a window smash without being hit?

Got home last night to a smashed window. The thing is, it was the sliding part of the window that was smashed and because the window was open, it was protected by the outside fixed pane of gl (which was still intact). It looks like something has hit it from inside the house. Nothing is near the window which might have hit it. No one was home either to throw anything at the window. The venetion blinds were all the way down and resting on the sill but they were turned to the open position. In front of the window is a long bath so you couldnt accidentally fall against the window. I cant find any reason why it smashed. Any ideas?