Tell you what, there's something screwy with levies in this game.
Other nations can pull up to 3000 men out of nowhere. I checked this guy, he was bankrupt and had taken 300 gold from moneylenders, and had 300 men after a long rebellion. DoW him. War is going in my favour, until he manages to pull TWO 1500 man armies from NOWHERE! No allies he could have dragged in, no money or anything. He should have LOST what remained of his troops after I attacked them.
I cheated because like hell I'm letting the game dick me with massive armies from literally fucking nowhere.
Could it have been holy order type things? I think they require just piety, though I'm not sure what would happen if you couldn't pay them.
Also "random" (maybe) events. I've had troops appear in my capital province during a war, 'cause apparently I was awesome enough at the time. Then again, I've also had my opponents get troops, so... maybe not.
Edit:
This gaaaaaaaaaaame
I get assassinated a lot.
Stop pissing off vassals?
The only time I was assassinated was because I had high tribal authority as an unreformed pagan.
Also set the spymaster to uncover plots 24/7
Also make sure your spymaster likes you. Whenever I get to be the spymaster of my liege, plots generally start at over 100%.
You can use that to your benefit though. The only character I've ever been assassinated as I manufactured it because I wanted his brother to inherit.