EE pulled the Tractor Trailer w/container up to seal placement rack. When on top of the rack there is a ""trap-door' in the catwalk. EE said EE placed the security hook to trap door to prevent it from falling. As EE was placing the seal lock on the manhole cover, the trap door fell down. the trap door hit EE in the back of EE's head pushing EE's forehead into the catwalk.