If any empire has >100 planets and >2 sector capitals, a sector capital can randomly secede from the empire. This is unpredictable but essentially unavoidable except for players in the Law & Order doctrine, who have a reduced chance of secession combined with a special unit (Imperial Guards) that deters secession.
When a sector capital secedes, it becomes the imperial capital of a new NPC “secession empire”. The act of secession is treated as the new secession empire conquering the sector capital. This will cause some of the player's planets within 250 LY to defect to the secession empire.
Secession empires behave like NPC warlord empires; this means they basically just sit there doing nothing.