Why does our dog seem to get angry, or at least excited, after making his evening deposit?

We've trained our shih tzu to go to the bathroom on newspapers in a litter box (believe it or not) when he's inside the house. Each night like clockwork, we'll hear him walking on the papers and we know he's going, and 30 seconds later he'll be suddenly growling and running around the house like a mad man (same reaction after a bath, but I think alot of dogs act like that after a bath). If you get in his way, he wants to fight. He's always done this and we refer to it as his "poop run." I originally thought it was a medical issue and was worried he might be in pain or something when he went, but he doesn't react this way when he poops outside, or even when he goes inside at earlier times in the day. Just at night. I'm clueless here. Does anyone have some insight they'd like to share? Thanks in advance!