Were you asked to choose between two allies when they go to war against each other? That could make you an unreliable ally for one of them.

But AIs love to break alliance without no reason. So the unreliable ally may be them, not you. Diplomatic relationship in Shogun 2 is the same number that goes both ways, so even if you are a saint to your ally, your ally may still hate you because it behaves like a jerk.

For example, when an AI attacks its ally, all clans in Japan acquire a big negative modifier "dishonor treaties" to the aggressor. I wonder whether Ouchi is a unreliable ally to some other clan, and somehow that penalty is now applied to you as well. Is Ouchi's relationship with other clans suddenly turning red, too?