Thanks Rich for correcting me. Here is an interesting link explaining the intricacies of making an appeal.
The Appeal Play
Pitchers are taught to step off the rubber to make an appeal to prevent the possibility of a balk call that would negate the appeal, but it is indeed not a requirement. Since dropping the ball is about the only way to balk in that situation (assuming Bob Davidson does not detect an irregular heart beat or an untimely drop of sweat), a pitcher does not risk much by doing it the way Pettitte did. Perhaps I missed something - maybe the Astros made a throw down to second, constituting an intervening play which would have negated a chance at a subsequent appeal on the runner missing the bag at first. Could that have done it? In my years of watching I have never seen an appeal by tagging the runner while he was standing on a base. It seems the throw is always made to the base in question.