Does it activate if you are not watching a movie, but just leaving it on the desktop for instance? Usually a screensaver will not come on when you are watching a video, as that would get very annoying if you had to move the mouse every few minutes to keep it from activating.
My feeling is when the movie is done, the video player is still running so it's preventing the screensaver from coming on.
My feeling is when the movie is done, the video player is still running so it's preventing the screensaver from coming on.