I don't think there's any scripting to it. The Pahlava start out with an army right outside Asaak, and they start out positively hating the AS, even though they're allied, so they almost always attack immediately. But every so often, they'll decide not to. I've seen it happen in some of my games too.
I think the fact that you're playing the Ptolemaioi may have something to do with it as well. As I recall, the Ptolemies start out being at war with the AS, Pontos, and Baktria. Because you're the human player, both Baktria and Pontos consider you to be a much greater threat, even if you aren't anywhere near them, and thus remain in solidarity with the AS. However much the Pahlava and Hayasdan may dislike the AS, if Baktria and Pontos aren't willing to join them, then they won't attack.
I'm willing to bet that if you make peace with Baktria and Pontos, the Hai and Pahlava are going to start eating the AS up.
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