OK, I KNOW the game is intentionally fucking me over. Vassals decide to revolt. I can deal with it.
Except that EVERY FUCKING ARMY ON EARTH seems to descend on me suddenly. They're fighting MY LIEGE. WHY THE FUCK ARE THE SUDDENLY BREAKING OFF A SIEGE TO TRAVEL HALFWAY ACROSS THE FUCKING ARABIAN EMPIRE TO ATTACK MY ARMIES!?
I'm not even fucking joking. An army sitting in Jerusalem decided 'Oh fuck THAT guy!' and sent their soldiers at me. I'm able to deal with them as they keep coming one by one, but it's still depleting my army and I REALLY do not need that given that the rebels have up to 1000 more men than me at present. I say at present because they keep summoning troops from fuck knows where. They don't have any mercenaries employed, but every so often 200-800 men will just pop up in one of their provinces (it's not the event that spawns them, either. They just appear without any kind of popup happening) and reinforce their army. I'd be fine if I could get them to attack me in a mountain province, but the shits are refusing to.
I've actually had to come off because I was getting so fucking pissed off at the game deciding to try to dick me over.
Sounds like your Vassals are replenishing from their levies. Hm, are your vassals and those fighting your liege hostile to one another?* If so, let 'em fight it out while your army sits on a mountain out of the way (with you in command, so you can't be captured if your holding is sieged). Previously, armies from levies would disappear if the levied province was captured, but now they remain.
What are your Vassals revolting over? Independence is easy: Let 'em have it, and reclaim them piecemeal with the claims that you get (if it works like I recall it working, things sometimes change). A lot of the other revolts are significantly more painful (Elective succession and install pretender, primarily, although with the new DLC, "War Committee" is also a pain).
*EDIT: Just remembered, that when subjects revolt, they become subjects of the next higher liege. Hence your revolting subjects (if you have the Old Gods expansion) are at the same level as you, and thus they should be just as vulnerable to the folks attacking your liege as you are. By which I mean, the leader of the revolt is the same as you.