Well I myself was meeker traveling alone than I would be in a group of friends. It was observing this in part that led me to my hypothesis. I'd like to think I wouldn't become rude with sufficient friends around me, but I haven't tested that. I did observe that the rude groups were ruder in other countries than I observed groups of the same nationalities being in their own countries.

Still, you could be right that tours select for rude tourists. The large, rude groups were more likely to have designated organizers. The rude Americans on the train didn't.