For rebellion is as the sin of witchcraft, and stubbornness is as idolatry and teraphim. Because you have rejected the word of Yahweh, he has also rejected you from being king.
Now he has shown a pattern of disobedience up to this point. He has become self-willed, doing his own thing. So the prophet is coming and warning him. This to me is very significant, because God does seek to warn us from our self-willed path of destruction. God doesn't just let you trip off into the path of destruction, without coming and giving you fair warning, oftentimes, repeated warnings.