SCRAPER WAS BEING PUSHED BY DOZER. SCRAPER OPERATOR WAS LOADING THROUGH HARD ROCKY DIRT WHEN HE RAISED THE PAN SLIGHTLY CAUSING THE SCRAPER TO PULL AWAY FROM THE DOZER. WHEN DOZER CAUGHT UP, IT BUMPED THE SCRAPER HARDER THAN USUAL CAUSING THE OPERATOR TO JOLT & STRAIN HIS BACK.