Vassals sometimes revolt when you try to imprison them or revoke their titles, especially if they're powerful--you can't expect them to take it lying down every time! You'll need to win the war to enforce your revocation. I remember when I, as King of Ireland, had a one-county count rebel and call in the King of England as his ally... Luckily, I managed to capture him in battle (bringing my warscore up to 100%) just as the first English stack of 14,000 arrived in Ulster.