I read a crash report that said that the aircraft completed a normal 30 degree banked turn to port, levelled out and flew level for a short time. The port wing then dropped suddenly, came level again and then dropped again hitting the ground. It was concluded that the aircraft was flying below the stall speed for its condition (nearly fully loaded).
What is going on here? Why should only one wing drop? Would the plane have levelled itself or could it be as a result of the pilot's actions?