Not so common on E53 but quite common on E39. Usually happens after some suspension job or similar where the knuckle is turned to extreme positions. The stump slightly looses C-clip locking but just barely keeps in place for a while. Usually it's OK just to push it back in and refill the gear oil. Personally I would remove the stump to make sure the C-clip was not broken and a piece of it is inside the diff where it could cause extreme damage (and of course use a new C-clip anyways).

__________________
E39 530dA -02 M-Sport Messing metallic
E53 X5 3.0dA -06 Sport Stratus grey
E70 X5 40d -12 M-Sport Space grey