OPEARTOR WAS DUMPING OFF ON STOCKPILE WHEN THE BACK OF THE PILE SLUMPED OFF. THE END DUMP SLID BACKWARDS DOWN STOCKPILE WITH END DUMP COMING TO REST WITH ITS. FRONT WHEELS IN THE AIR. UNKNOWN TO THE OPERATOR A LOADER HAD LOADED A CUSTOMER FROM THE BACK OF THE PILE, CAUSING THE PILE TO SLIDE WHEN HE BACKED UP.