I've seen that happen when the Horde splinters, too. It seems like the game randomly decides how many successor-states there should be and randomly parcels out systems to them, but isn't capable of assuring they each have at least one planet too (which isn't surprising, since the Horde's success and ability to occupy/take over planets can vary wildly).I'm not sure it just happens with Spiritualists. I had a game where the Horde broke down into four unequal successor states:
IIRC one game that happened I declared war on them, took one empty system off of them, and the whole nation spontaneously collapsed because the game suddenly realized they didn't have any territory or population.