Saw this posted on a different riders forum:
When prompted, we got a few more pics:
Obviously, the rear lug bolts loosened up and fell out, causing the rear wheel to extricate itself from the bike.
Fortunately, the rider wasn't going all that fast, was ATGATT, and escaped without personal injury.
The rider said they'd bought the bike used a few months back and hadn't yet messed with the rear tire at all. So whomever did the last tire change clearly didn't torque the lug bolts down properly.
Note this is an R1100R, but I'm posting here because the final drive and rear wheel mounting procedure is similar for R1100, R1150, and K1200LT bikes.
From the R1100 BMW Service Manual:
Use no oil or grease on the rear wheel studs.
Hand-tighten the rear wheel studs in a cross pattern, then initially torque to 50 Nm (37 ft-lbs), then final torque to 105 Nm (77 ft-lbs).
Again, this is the same for the K1200LT rear wheel.
Newer R1200, R1250, K1200, K1300, and K1600 bikes have an inverted Paralever drive with a wheel flange, so they have different torque specs. Note that this wheel mounting flange can be damaged by over-torquing. There is an official BMW Recall Notice on this issue.
From the K1600 BMW Rider's Manual:
Tightening sequence: diagonally
Torque: 60 Nm (44 lb/ft)
I can personally vouch for the fact that the K16 rear wheel can loosen up if the lug bolts are not tightened properly. Fortunately, I noticed that the bike felt squirrrely, pulled over into a safe place, and was able to re-torque them correctly (I carry a lot of tools onboard).
So the moral of the story is: Check your torque values (front and rear wheels), and always wear your gear...