EXCAVATOR WAS LOADING A LARGE ROCK INTO INJURED'S TRUCK, FROM THE SIDE. SINCE EXCAVATOR WAS LOADING FROM THE SIDE, HE COULD NOT PLACE THE ROCK INTO THE BED, HE HAD TO DROP IT. THE WEIGHT OF TH E ROCK CAUSED TRUCK TO ROCK. INJURED ATTEMPTED TO HOLD ON AND HE FELT PAIN IN HIS BACK.