PERSONNEL WERE UNLOADING A SHIPMENT OF BLASTING ACCESSORIES THAT ARRIVED THAT MORNING. BOXES OF BOOSTERS (65 LBS) WERE BEING STORED AND STACKED. EE WAS REARRANGING AND STACKNG SOME BOOSTERS WH EN HE TWISTED AND HEARD A LITTLE POP IN HIS BACK SO HE WENT DOWN TO THE CLINIC.