There is something wierd going on with vassals or maybe I just missed some part in the changelogs again

:
It's the 2nd game now that my vassal canceled all the ressources he "tributed" to me, but didn't cancel his vassal status to give me the option of declaring war or having piece.
Then I demand one of the ressources as tribute and the vassal instantly declares war at me!
I thought, that vassals can only cancel ressources by cancelling their vassal status, which gives
me the possibility to declare war.
Edit:
I played ahead, fought the war until I could vassalize my former vassal again and demanded a ressource in the same round, which again led instantly to war.
My other vassal doesn't either cancel ressources nor show any buggy behaviour at all.
It might be possible that in both games I've experienced this, there was a gold per turn tribute included with the peace treaty and capitulation, which then is maybe canceled after 10 turns leading to this buggy behaviour.