It's probably caused by a broken or missing casing spring. The first picture is the movement casing ring and spring, the second is how the spring fits onto the ring.
The caseback screws down onto the spring and prevents any movement. If it's broken or missing there will be little support and will allow the movement to move slightly when the crown is unscrewed. When the crown is screwed in, the spring on the split stem pushes the movement and ring against the opposite side of the case and prevents any further movement.