[ben@tirerack]

Did you notice any irregularities or patterns to the treadwear? That would be my guess if the problem is specific to when you rotate back to front. I know BMW often does not recommed rotation even on cars that are not staggered, as the alignment settings sometimes cause irregular wear even when the alignment is in spec (inside edges of rears wearing due to negative camber is a common example).

Have you had the the balance checked on the tires? Possible that a weight may have fallen off or a tire developed some type of flat spot while in storage.

All of this is hypothetical, and there may be something I'm not thinking of, but those are the most likely thinks I can think to check on.