Diplomacy is broken / AI is too aggressive
Post patch, how other nations feel towards you doesn't seem to matter. Your bestest buddy, who is Very Friendly towards you, and who you've been peaceful with the entire game, and have had trade relationsh for the entire game, will declare war on you, completely out of the blue, practically as soon as you have an adjoining border.

If Very Friendly nations declare war on you, then the entire disposition system is meaningless.