EE WAS WORKING OFF OF A 8 1/2' SCAFFOLD PLATFORM. HIS FALL PROTECTION LANYARD WAS ATTACHED TO AIR CHUTE VALVE ASSEMBLY LOCATED OVERHEAD. HE AND CO WORKERS HAD JUST FINISHED DETACHING AN AIR SL IDE FROM THE VALVE ASSEMBLY WHEN THE ASSEMBLY FELL DOWN FROM ABOVE. BECAUSE EE WAS TIED OFF TO THE ASSEMBLY. IT PULLED HIM OFF THE SCAFFOLD ONTO THE FLOOR.