Disney added the Rock 'n Roller Coaster Ride in 1999 which makes it seem like your touring L.A. in a limo. During the ride people are shown a video of Aerosmith during one of their sessions.
In the video at about 3:34 Steven basically does the "Shocker" gesture in the video during the ride.
Disney found a way to alter it so he's not giving that particular gesture anymore.
Source 1 and 2