It's usually the case that The Witness becomes The Narrator in the next scene. However, during Final Horror both of these roles carry greater narrative power than the roles of Watchers and have comparatively greater influence over the way the plot is wrapped up than in other scenes. This is caused by the fact that the Narrator can almost single-handedly decide the nature of the horror and it falls to the Witness to decide how the main character reacts, forcing plot into a direction Watchers have relatively little influence over.
It only makes sense to make it up a bit to the Watchers (plural, as in games with more than 3 players both Narrator and Witness roles are in the hands of former Watchers). Effective double-rotation also has an additional bonus of landing the Narrator role into the hands of a player who was a Watcher the longest. You could reason that a double-shift could be used in all scenes, but in even-numbered groups it would mean players only get to experience one of the roles.