Noticed something a little odd ~ if you play as Hayasdan or Pahlava the other faction will attack you despite having no common borders.
Is this intended or an AI quirk/bug?
Does it attack you by blockading one of your ports?
I get the same thing from the Sauromate and Lugiones in my Pergamon campaign. Both of them send armies through the territory of other factions to attack Sinope. I think it's just the AI having a vendetta against the human player (M/M settings, btw).