Wait, didn't you just describe it here?
Person A: (perhaps unwittingly) displays a signal that they're not in Tribe T.
Person B: You're horrible! You're not Tribe T!
Person: I'm not horrible! But if you don't want me, okay then, I'm in...whatever the other tribe is.
Isn't this completely classic coalition-building behavior? You see who your enemy is by seeing who is hostile to you, then you join up with the other enemies of your enemy?