Regarding snowballing, when it comes to games in a similar genre as bannerlord, they'd often just suppress it with things like threat level, aggressive expansion debuffs, etc. All of those are supposed to provide a mechanic by which the other AI kingdoms would group up to beat up the kingdom that has grown too strong.
I suppose in Bannerlord, the concept which could apply for a medieval period would be a coalition, defensive pact, or if there was religion, crusade or jihad.
From my experience, it's more that the AI is unable to mount a good defense against a numerically superior enemy. And from all the sieges I've played thus far, it doesn't seem like it's possible for lords to intervene in a siege, unless they're already in the town/castle's garrison. That is what I believe is the main problem, as well as a wonky siege defense AI, that sometimes just runs straight into your troops that got on the walls without even trying to fight them.
And it also seems to me that AI is more keen on attacking than defending, since the armies that they form are very rarely used for sole purpose of defending a besieged town or castle. At least from the several sieges I've done so far, there has never been an enemy army sent to relieve the siege or just fight our army to thin out the numbers.
...Wait, religion is not a part of the game, but it does sound like something that could be added, though.
There's still a lot of work left for the devs to do, so can't wait for the next patch. Keep up the good work, guys!