When I saw the replays on the news, I had some doubt in my mind as to whether Pierzynski had touched the plate during the initial collision or not. So making sure (by slapping the plate when he did) was absolutely the right thing to do. I will admit that he did it with more zest than needed, but it was certainly no worse than many other "celebrating getting to home" things I've seen on a routine basis (jumping a bit to land with both feet together on home plate, etc.).

My guess is that Barrett took the enthusiastic hand "plate slap" to be an "In your face, I knocked the ball away" statement by Pierzynski, thus triggering Barrett's "I didn't have the ball..." comment and punch.