TRUCK ""A"" WAS DUMPING HIS LOAD WHEN TRUCK ""B"" BACKED IN BESIDE HIM. TRUCK A'S TAILGATE FROZE SHUT AND HIS LOAD DID NOT COME OUT, HE PULLED FORWARD, THE TRAILOR ROCKED AND THEN THE BOTTOM JACK PIN BROKE AND THE TRAILOR FELL OVER ONTO TRUCK B'S DRIVERS SIDE. THE TRAILOR TRAPPED INJUREDS FOOT ONTO THE FLOOR BOARD.