Motoring YAW servo?
NEVER take your feet off the pedals in a Bell medium.
Know someone who had the habit of flying with feet off the pedals.
Pre-landing check - Force Trim off. (Feet off pedals)
Pedal motors to the stop! Direction of travel now visible through greenhouse window!
I think it has something to do with the way the servo is mounted vertically and if the input is not balanced.
Good check is FT OFF and running at 100% flat pitch. Feet near pedals to stop subsequent movement and tap each pedal direction (L/R) in turn to see if it motors on it's own. If it does just stop it with you feet and do not allow it keep moving of course. You may be surprised.