EMPLOYEE WAS PACKING TYPE S ON PORTLAND WHEN TWO BAGS FELL OFF PACKING MACHINE ONTO THE CONVEYOR BELT BELOW. EMPLOYEE TRIED TO HOLD BACK ONE OF THE BAGS WITH HIS RIGHT FOOT TO PREVENT THE TWO BAGS FROM PLUGGING THE TRANSFER POINT. WHILE STRETCHING DOWN AND EXTENDING LEG TO HOLD BACK THE BAGS, HE FELT A POP IN LOWER BACK.