Rockee may feel intimitated by larger dogs and thus acts aggressively towards them!

And Thunder was in a new house that he was not familiar with. Even if he's visited there before, he doesn't live there. So along with the car ride there (it COULD have stressed him out??) I'd bet my money that that's why he had an accident -- he was in an unfamiliar house Jamie did the same thing when we were out of town visiting my fiancée's parents.