Some years ago I was standing in a hole in the backyard, digging a pond. I had paused to rest and was looking at my house when I heard an odd crack and a big window (the stationary part of French doors on the back of the house) began cracking. I watched it over several minutes as the entire thing cracked. Looked just like yours. It was fascinating and horrifying to watch, and we could not fix it for years. Finally replaced it a couple of years ago.