What difficulty setting are you playing on?
This sounds like the programmed, constant deterioration of relations on H and VH setting that was in M2TW, absent bribery or something else to stop the slide. At M difficulty relations would stay static, at Easy they'd slowly keep improving. If you're playing at M or Easy, that blows the theory.
This could also be the targeted province effect, if you're sitting on a province the AI needs for a win condition. There are a lot of overlapping "gotta have it" provinces in this game, and the world starts in a more developed state than M2TW or RTW.
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