Re: Question About Secession
Your political opponent is the Disciples of Zeus, but the seceding party is the Disciples of Poseidon. As the player's faction expands, extra rival parties will appear, it sounds as if a new party recently emerged.
It sounds like you're wondering if the secession by a recently emerged party was linked to the purges. Your message saying that they had gone into hiding and would reappear sounds intriguing. I haven't seen this message, perhaps you have discovered an aspect of internal politics which I haven't seen.
You wrote that this occurred in a region where you hold political favour. The regions held by different parties can change over time, for example when the influence of parties changes or a new party emerges.
In my experience, if a party which secedes doesn't have armies provides by the player, the game provides them with armies.
If you would like to avoid secessions, you might find the discussion on another thread useful.