This happened to me several months ago as I went over some cobble stones in the center street. It went from 3rd to 1st. I think it might have been convenient in my situation where as if you pre-loaded for a down shift and it was at the right mesh point and went down cuz I was going slow enough, and had the clutch in but still.... Now I probably do have a busted tranny due to a chain jump on the freeway exit the other day. This little auto shifting situation may have been an advance warning my tranny had problems to begin with and the chain thing was the final draw....