Wow, that's really bizarre, never heard of such a thing. Normally the motor, when unpowered, has enough resistance to prevent that from happening. Since all 4 motors are identical, you might try swapping the Z motor with the X (easiest to get to) and see if that fixes it? If the problem goes away, then you know it was the motor. If it doesn't, then it is the drive circuitry on the mighty board.