I think it seems to be an area where dust etc. can get in and cause havoc. In my case, it appears to have been more serious, as the track was replaced. Perhaps the debris had caused wear? To answer another point, yes, I suppose it was quick but it seemed frustratingly long to me as I'd only had the camera a week. If it had taken any longer, the dealer was prepared to loan me an M7. Good service I think.
Out of interest, it cost the dealer £40. I don't know how much I would have been charged had it not been under warranty.