Not sure, it's happened to me twice so far. Same situation both times. Reversing into my garage, it's a tight squeeze, without seat belt on. First time I thought I had hit something, but not that then I thought it had slipped out of gear, but thinking about it i wondered if it could be the electric brake activating. Never had it happen in normal reversing situation with seatbelt on. Also it from doesn't happen everytime I reverse into the garage so maybe it's a slight speed difference that activates it.