The game I've been playing is with a xenophobic empire, and I've been doing a lot of purging. Sometimes, when the planets I'm purging are on my borders, I'll set up a frontier outpost in the system so that I can hold it before I terraform and then repopulate. However, after the purging is complete, for whatever reason, my frontier outpost disbands. I'm not sure if this is intentional or not, but it seems like a bug.