When you score a TD, the word "Touchdown" appears. When it first appears, press the following button combinations. Note: This could also get you an 15 yard Unsportsmanlike Conduct penalty.

L + B: Player spikes the ball, points to the crowd, then beats his chest.
L + Y: Player bows to the crowd.
L + X: Player spikes the ball, then shrugs his shoulder.
L + A: Player does Heisman Trophy pose.
R + B: Player will dunk on the Goal Post.
R + Y: Player will punch the ball towards the crowd.
R + X: Player will kick the ball into the crowd.
R + A: Player will throw the ball to the crowd.