The AI is scripted to commit forced invasions. It's stupid and a poor design choice, IMO, but it has been present in all TW games unfortunately. Check out this thread for a good discussion on psycho AI and possible ways to offset it:

https://forums.totalwar.org/vb/showthread.php?t=75496

Also, you may want to delete all of the tall_poppy triggers from the descr_faction_standings file to reduce the senselessness of the AI, and to prevent the game from punishing you just for being successful (instead of the other way around, that you get rewarded by an enjoyable game).