Answer:
not sure but it happened to me, car was stuck in third so i had it towed home. the tow truck (flatbed) backed it into my garage and since it was in gear it didn't roll back. i got in the car to operate the break while the tow truck driver jerked the flatbed while the car was on an incline. out of nowhere it started rolling backwards. i slammed on the breaks to maneuver it into the garage and when i checked again the shifter moved freely.