INCIDENT OCCURRED AS EMPLOYEE WAS IN PROCESS OF DISMOUNTING A CAT 793 OFF-HIGHWAY TRUCK; HE'D JUST COME DOWN TRUCK'S FRONT STAIRWAY & WAS TURNING AROUND TO CLIMB DOWN THE ATTACHED FLEXIBLE LADDER. AS HE WAS TURNING AROUND, HE STRUCK HISRIGHT ELBOW ON HANDRAIL, CAUSING HIS FOOT TO SLIP OFF THE STEP, RESULTING IN HIM FALLING BACKWARDS OFF THE MACHINE.