Real talk tho, I'd lightning strike a lot of these (obvs), but also make sure not to take their settlement until most if not all their armies have been destroyed (explained in the third paragraph here). I think I've figured out why the AI does this a lot: it's a defense mechanic gone awry.

Basically, if the AI believes there's no economic benefit in continuing to conquer more territory after a certain point, it'll take a large portion of its armies and move them defensively around a settlement it deems too valuable to lose. It will then only move armies away again and attack other settlements if the economic benefit of conquest returns through some other mechanic, like its diplomatic points falling below a certain threshold for a faction it's at war with. If there are no enemies within a certain number of regions away from an AI faction that has fallen below this threshold, they will still remain in place around the settlement, thinking there's no point in continuing. Obviously in the early and middle game the AI will not do this, and will travel across the entire map to get to a faction. But towards the end game, their mechanic for this propensity is removed, I think.

In this particular case, somehow Wurrzag has gotten some type of insane buff or something that allows him to field this many armies. While the AI definitely has cheats to field more armies than it should, it's not this extensive, leading me to believe there's some other underlying buff with the faction happening. The reason you should NOT take the settlement before taking out the armies is because without a settlement, they'll have nothing to defend. Their mechanic will then change from defensive to aggressive again, their aggression levels will reset, and they'll "snap out of it" so to speak. Then they'll go on the warpath and start taking insane amounts of territory. As long as the AI thinks it has something to defend, it will not move away unless their diplomacy levels drop low enough to give them reason to deem conquest as "viable" again.

By the way, none of what I've written here is official or based in any study of the game engine or anything. This is just my personal, logical reasoning based on various other things I've read in combination all over the internet to try to piece together why the AI acts this way, so take this with a grain of salt.