Because half of the time they were declaring a war they can't win, and the other half was when they could, but lost anyway, and there's nothing I can do about it because they were the aggressor.
You're right to mark (CN?) as you have. That sort of behavior is very widespread (easily seen outside of situations involving CNs as well) and is a huge problem with the game right now, one that's gotten significantly worse as a result of the last AI tweaks.The (CN?) AI is ridiculously bad at dealing with the Guerilla warfare of inferior nations. They rarely properly split their superior forces, and if the two allied tribes are far enough from each other on the map then the two armies of the tribes will keep sieging and (re)occupying undefended provinces and roughly the pace at which the CN does the same. That leads to an infinite goose chase until war exhaustion forces the CN to peace out in an unfavorable deal, I suppose (never quite understood how those peace deals offering provinces to the natives came together, as it never quite looked like my CN was really losing, just ... not winning. Maybe they lost their capital, lol).