When do Sectors Purge?

[Rabbit Burgher]

So, under settings I can allow a sector to purge. In what situation will they do this?

 

[Baro]

Once 1.5 comes out.

 

[Mackus]

So, under settings I can allow a sector to purge. In what situation will they do this?

If you're xenophobe, they'll purge aliens, but only if there are also pops of your own species (or robots) on the planet.
If you're collectivist, its kinda less clear, they'll generally purge pops that are very unhappy/in hostile factions/have wrong ethics.
If you're both xenophobe and collectivist, they'll purge both.