TWO EE WERE ATTACHING A SET OF DOLLY WHEELS TO THE PULL AREA A LOADER. BY USING A BACKHOE TO LIFT DOLLY WHEELS INTO PLACE. AFTER PULL PIN WAS SET, OPERATOR MOVED BOOM OF BACKHOE DOWN IN ORDER TO SLACK THE CHAIN TO UNHOOK. WHILE UNHOOKING, OPERATOR BUMPED BOOM LEVER, DROPPING THE BUCKET ON THE MANS RIGHT HAND.