Could also be cold jam. When the lid is open, the air inside the jar would be room temperature. As soon as you put the lid on it, the air starts cooling down be

Could also be cold jam. When the lid is open, the air inside the jar would be room temperature. As soon as you put the lid on it, the air starts cooling down because the jam is cold, which would lower the pressure, and then the outside air would push the lid down, turning it. Or it's just gravity.