I have the same issue. I never have sustained any major damage to the car. My rear bumper looks exactly like the picture previously posted. I took it to bmw of manhattan today. They said that my bumper needs to be replaced without even looking touching my car. The service specialist told me it would cost $928!!!! I could not believe it. I tried to push the tabs back in. I saw one of the plastic tabs broken, and the others were just dislodged. I was unsuccessful after fiddling with it for a half hour.
I called bmw of north america, gave them my VIN, and reported the problem. They said they never have heard of this rear bumper issue before and that I should take it to another dealer for a second opinion. I suggest that anyone having this problem report it to bmw of north america (1800-831-1117) with your VIN. It seems ridiculous to me that little or no trauma to a vehicle can cause a problem costing near a thousand dollars!!!! If enough people bring this to BMW's attention, hopefully this can be addressed.