You could be on to something there, every GB or France game I've played the Cherokee go nuts on Spain in Florida and capture it. As GB that's of course nice because I get free province without starting war with Spain.
Even the Hurons always start war against GB if I'm playing France, GB or Spain.
Any idea will this berserk behaviour be triggered if you are any other nation (lets say UP) and conquer 1 province from mainland America?