THE YARD LOADER OPERATOR WAS LOADING A HAUL TRUCK IN STOCKPILE AREA. WHILE LOADING, A CONTRACTOR PULLED HIS TRUCK BEHIND LOADER. LOADER OPERATOR BACKED AWAY FROM HAUL TRUCK TO GO FOR ANOTHER B UCKET W/O SEEING THE CONTRACTOR. WHEN HE DID SEE THE CONTRACTOR IN HIS MIRROR, HE SLAMMED ON HIS BRAKES TO AVOID HITTING THE CONTRACT TRUCK. HE STRAINED & PULLED HIS BACK MUSCLE FROM THE STOP