Exploration kills?

[Maltz]

It seems to me that the AI will not hesitate to break alliance if they are surrounded by allies, epsecially if you are one of their allies. This is probably your case with Ouchi, who are allied with Shoni and Mori. So if Ouchi do not start fighting Otomo (or Otomo dies too fast), you might become the victim of their script routine.

When AI is surrounded by AI allies, they will either break one alliance and get the "dishonor treaties -45" modifier with every clan, or launch a naval invasion to somewhere far away. Which way the AI chooses is probably controled by personality. Imagawa seems to always go for the latter. While Hojo loves to backstab.

Another very mean feature in the game is that when an AI clan breaks an alliance with you, there is no message. You have to manually check the diplomatic status to know the alliance is broken and an attack could be imminent.