EE WAS LOADING 777 CAT END DUMP TRK WITH A 992 LOADER(60053)AT THE PROSPECT AREA WITH ROCK & DIRT. THE EE ALLEGEDLY TWISTED & JERKED LOWER BACK WHEN A ROCK ROLLED UNDER THE LOADER FRONT TIRE WITH THE BUCKET APPROX 2/3 FULL IN THE RAISED LOADING POSITION. THE LOADER LEANED SEVERLY TO THE LEFT (SOUTH) CAUSING OPERATOR OF THE LOADER TO QUICKLY TURN THE LOADER TO THE RIGHT.