THE OPERATOR WAS ENTERING OUR STOCKPILE AREA AND CROSSED A DRAINAGE DITCH. HE HAD BEEN THERE EARLIER IN THE DAY TO DRAIN WATER FROM THE STOCKPILE AREA. HE WAS TRAVELING APPROXIMATELY 15 MPH WH EN HE ENTERED THE DITCH. AT THAT TIME THE WEIGHT OF THE LOADER SHIFTED FROM LEFT TO RIGHT WHICH FORCED HIS BACK TO SHIFT LEFT TO RIGHT THEREFORE CAUSING A MUSCLE STRAIN IN THE LOWER BACK.