The AI DOESN'T Make intelligent decisions the whole reason this snowballing exists is because the AI is absolutely brain dead. It runs around with no troops getting caught by bandits, declares war when losing all their current wars, seizes territory in the middle of enemy territory rather than adjacent to their own and rides into combat with a tiny number of troops only to all get stackwiped, or worse they run up to an eneny sieging a town or castle and bounce back and forth until the siege ends - at which point they all get captured and imprisoned.
Additionally none of the other nations react to the growing threat, which is why one nation even manages to streamroll in the first place.
You like stupid and non-functional AI, not smart AI.
And it is concerning when a DEV says that steam rolling could still happen. It's a major step back in design. Look at Viking Conquest, that could never in a million years happen, I know because I was over 3,000 days in a few times.