One of your brake pads is sticking on the shelf in the caliper that allow the pad to move in and out relative to the disc.
Had the same issue with my 525. Reverse and clunk, but only the once for that day and it didn.t mater if the car was reversed first thing or last thing in the journey.
Simple cure is to remove the pads, clean up the caliper. Apply a very small amount of copper grease to the locating lugs on the pads and re-assemble.
I believe it was a build up of brake dust caused by cheap pads and an auto gearbox so the brakes get used more. In the end I just learnt to live with it, the e34 only did journeys in stop-start traffic so the pads were constantly creating dust.