EMPLOYEE WAS IN THE LEAD SCRAPER PUSHING THROUGH AN OVERBURDEN REGRADE AREA. THEY HAD FINISHED LOADING AND WERE GOING DIRECTLY TO THE FILL AREA TO BEGIN DUMPING THEIR LOADS. EMPLOYEE WAS BUMPED BY THE REAR SCRAPER AS THEY WERE COMING UNHOOKED. HE WAS LOOKING BACK TO ASSESS THE SITUATION AND DID NOT RECOGNIZE A ROCK IN HIS PATH. HE DROVE OVER THE ROCK WHICH IMPACTED HIS BACK.