Wow. You must have really abused your colony. Colonies start with something like +10 relations with their mother country.
Or were you the English and spawned America led by George Washington? Was the year of renunciation 1776 by chance?
It wasn't me; it was an AI. Lincoln spawned Ragnar as a vassal, Ragnar chose a different religion from Lincoln, and less than 15 turns later he was a free state. Funnily enough, Ragnar still has the +10 bonus so is pleased with Lincoln despite religious differences, but Lincoln now has nothing to give him bonuses from Ragnar and is straight up annoyed with him.
Shows how crazy Ragnar is though; he had three 1-pop cities, no workers, and almost no units and he "renounces his protection." Ragnar, you're SUPER weak. I'm very strongly thinking of DoWing him/