Sagging rear suspension??

[TTBear]

My wife and I have noticed that the rear suspension on the X6 is sometimes "bottomed out" - even after short periods of sitting in the garage. On one occasion, it was only the passenger side. More recently, it was both sides.
The suspension looks completely slammed, with the upper edge of the tire almost tucked in under the lip of the bodywork.
Upon starting the car, the supension raises up again, as I expected it should, with a self-levelling suspension. It of course only occurs on the rear suspension.

My question is - should it be settling down that low ever, at all? Is this normal?

Any insight appreciated, thanks.