Are they joining factions or rebellions when you try to imprison/revoke something? In the latter case maybe they like the person you are going after a lot?
The latter, I'm triggering a rebellion by trying to revoke a county from a bitch of a countess. It was a good idea, them liking her even more than me. But no, they don't even like her, the three high opinion vassals that join her have -10 opinion of her, where they all have 85+ opinion of me, 100 in case of my sons in laws.
It's also not a faction thing (the three are not in any factions), or a religion thing (she's the one with a different religion here), or a culture thing (different culture as well), or traits (the three have different traits, but none of them are ambitious, deceitful, arbitrary or something like that), or hooks (none of them have any as I just gave them land). I had a thought that maybe it's a family thing - AI sees themselves, their spouse or their kid somewhere in that line of succession of my kingdom, and will try whatever stupid thing it can to push that name one more place to the top. But no, that does not appear to be the case either, I tried disinheriting my daughters before triggering the war to remove that motive, and my idiot sons in laws happily join the rebellion all the same. And this would not explain not-family high opinion vassals that do the same thing.
I guess some people just want to see the world burn? Or it's bugs, can't ever rule that one out with Paradox.