Mysterious broken pitcher

   My wife collects those old potbellied white clay pitchers that are so commonly seen in Victorian pictures and even older representations. The pitchers are definitely bottom heavy. As they are a favorite of my wife's. it would be most unlikely that she would place one where it would be in danger of being broken. Our kitchen has a doorway that opens into a short hall (no door) off to the left are the rear stairs to the upstairs, straight ahead brings you through another doorway into the diningroom and off to the right takes you through a doorway (with door) into the downstairs bathroom. A wide shelf, well secured to the wall and top of the door, runs across the top of the doorway out  of the kitchen. That shelf is solidly anchored on both ends as well with heavy brackets. On that shelf my wife has placed one of her pot bellied pitcher plus two other pitchers and a couple of chicken figures. I was working upstairs in the front bedroom doing nothing heavy, I might mention, and my wife was painting details on the bathroom wall. Both dogs were sacked out in the dining room. Suddenly a loud crash was heard, bringing us all from different directions. The white pitcher was in pieces on the kitchen floor . Nothing else on that shelf was even moved and the shelf is still very solidly fixed. Only that pitcher which was bottom heavy (surely too bottom heavy to tip) was off the shelf and broken. There appears to be no explaination for this. So what happened?