The Apple TV player will automatically switch back to the external screen mode when a video playback concludes, or when switching from a video to a document. This means that the Apple TV Home Screen may be visible on the external display when switching between the two modes.
To have a cleaner interface, and prevent the Apple TV displaying movie or TV show previews, you can change the Home Screen of your Apple TV. With the Apple TV app selected, press-and-hold on the main Apple TV remote button until the app starts to wiggle, press the Play button for options and select New Folder, then place the Apple TV app in the new folder. Then if you default the selection to the folder containing the Apple TV app the only thing that should appear in the Top Shelf of the Home screen is the Apple TV logo.