These little plots that say some random mayor is plotting treason or has slandered you seem pretty arbitrary. There's no way to know what's actually going on. I think just like the event where the wife is pregnant with a bastard, you should be able to hire spies to investigate and find out if the character really is plotting against you. Right now it's just a random way to revoke titles from characters you don't like, if it ever even fires on them.