So, I was initially doubtful that Chaos Dwarfs would be involved- I'm not aware of any example in canon of them taking direct action against the Karaz Ankor, they mostly seem content to ignore their non-corrupted cousins- but I did think of a possible motivation. Might not be accurate, who knows, but I think it's more of a reason than "Some CD decided to fuck with the KA for shits and giggles".
So, that map up in the spoiler, right? Boney has stated that when K8P was retaken and Death Pass became safe to travel, a much larger portion of the Silk Road trade routed through Death Pass- caravans that might have tried High Pass or Peak Pass deciding to travel by Death Pass instead.
We know caravans often stop at Uzkulak- not that they'd ever admit it- and I certainly don't see many ways past Peak Pass where you wouldn't have to make a deal with Chaos Dwarfs to get by. Meanwhile, the route past Death Pass seems like the most likely where caravans would try to slip by instead of giving tribute or trading with Chaos Dwarfs.
So, my thinking is this- the Chaos Dwarfs at Uzkulak notice that they're getting a fraction of the caravans that they used to, and figure out that it's because K8P was retaken. So, they plan some retaliation. Maybe they just wanted to hit a convoy coming out of K8P, maybe they know about the Marienburg situation and want to get the Karaz Ankor in a war to weaken it, or maybe- as an appendum to the second option- they use the war as a distraction while they take out K8P.
At any rate, that's the only motivation that I could think of beyond "Screw the Karaz Ankor", and that generally doesn't seem to have been enough to get them moving.