More details:
It is not dependent on struggle status.
Off the start of a fresh run myself, within the first 5 years of the game start (867), I've had the same BARONY tier vassal fabricate a claim on my county title twice, despite the fact that barons cannot act on claims against county-level rulers (which I am) or form factions.
I'm losing 350 prestige every time the claim is successful, and I'm beginning to suspect that there is nothing that can be done to stop it, because perhaps the game does not detect that a baron could possibly be instigating a plot to claim the throne?
Both myself and the vassal are detractors of the caliphate, and he has a 40-positive opinion of me and is honest and trusting, so I'm not sure what his motives are other than being an ***hole and draining all my prestige. (and, again, he's a baron, so it isn't as if he can use the claim)