EMPLOYEE WAS BACKING UP ARTICULATING TRUCK TO DUMP A LOAD OF ROCK. AS EE WAS IN POSITION TO UNLOAD THE ROCK, EE PUSHED THE LEVER TO LIFT THE BED UP AND AS IT WAS GOING UP, EE FELT THE BACK TIRE SLIPPING SO EE TRIED TO PULL THE TRUCK FORWARD, CAUSING THE CAB TO LIFT AND FALL OVER. EMPLOYEE WAS WEARING SEATBELT. EMPLOYEE WAS SEEN BY MEDICAL DUE TO ARM REST CAUSING BRUISING.