A FIELD CONVEYOR HAD STOPPED BECAUSE A ROCK GOT JAMMED IN THE TAIL PULLEY CAUSING THE CONVEYOR TO STOP. AFTER THE ROCK WAS CLEARED FROM THE TAIL PULLEY, EE NEEDED TO SHOVEL OFF CONVEYOR TO GET IT TO ROLL. WHILE SHOVELING ROCK OFF THE CONVEYOR, THE BELT STARTED TO ROLL BACK CAUSING HIM TO GET PUSHED TO ROCK BOX.