Empire factions turning into chaos?

[Alestorm]

In my nurgle campaign I have pushed Slaanesh out of their lands and destroyed the Tsneetch, but Khorne has ventured down into the empire lands and then at around turn 40, Wissenland's flag turned into the chaos banner and they are now allies.... Did Khorne make them vassals or something? Im new to this mod so im confused

[Vette]

When religion level of Empire factions drop below certain level, each turn there is a % chance they will turn to Chaos. You can revert them back by locating and eliminating their faction leader.