In the scene where the first nanny becomes extremely angry, Cartman says "Yes! Let the anger come! Strike me down while you can!" This is a reference to Star Wars Episode VI: Return of the Jedi, when Emperor Palpatine tries to turn Luke Skywalker to the dark side by inciting anger in him.
The second part of the transformation (when Cartman's body shifts from different forms and he is banging against the walls in a hallway) is a parody of the movie Altered States.