Ultra heavy antique wringer jumps off shelf

This is one for the armchair sleuth. Yesterday at our store in Byesville, Ohio a seemingly impossible occurrence occurred. My wife's store is primarily a store for women who want to change their homes with new curtains, vintage furniture or Victorian bric a brac. In the store's main showroom runs a shelf, about five foot from the ceiling, around the room. What the original purpose of this shelf is unknown but we use it to display vintage items that just wouldn't look right out on the floor. Amoung these items such as old sleds, lamps, toys, was a very heavy old washday wringer. Originally this wringer sat on top of an old rustic wooden tub. The wringer was made of cast iron and wood. It had a large cast iron crank on it so the lady of the house could use it to wring the water out of washday clothing, etc. The wringer was placed on the shelf roughly two years ago by two people handling it. It sat solidly on the overhead shelf and was leaning against the wall of the store. Here's where it gets interesting. My wife was at the cash register going over some orders. I was in the work area just off the side of the rear sales area. Suddenly I hear a thunderous crash and my wife's exclamition. I honestly thought the store had been struck by a vehicle. I ran from the work area to the main sales floor. My wife was moving out of the cash register area toward the middle of the sales floor. "It jumped up into the air and fell off the shelf" she was saying. I could see the wringer was half on the floor shelf and half on the floor. "What do you mean, It jumped off the wall?" I asked. Going over to the wringer, we could see the handle of the thing had managed snag two packaged curtains when it hit the lower sales shelf. Keep in mind, this wringer is heavy, I haven't weighed it, but it must weight 40 or 50 pounds. It sat flat on the top shelf and was leaned against a wall. If the wringer had simply slid down the back wall, it would have still been on the shelf. The wringer was leaning BACK against the wall but had managed to fall face forward. A small piece of wood was chipped off the wringer housing, but otherwise it was fine. My wife said she saw it "jump" before it fell. If something had jostled it that severely I would have thought that much lighter in weight items would also have fallen or at the least moved. Nothing was out of place but the wringer. This is a strange one that common sense cannot explain. Anyone out there with a good explanation is welcome to comment.