Revolt

From the Galactic Civilizations Wiki

Revolt can be a blessing (when colonies join your civilization) or a curse (when your colonies leave your civilization). When another race's influence in a sector is 4x what the planet is producing, there is a chance the planet will revolt and join the other race. It is a bloodless way to conquer the galaxy and many revolts may be a sign of a coming influence victory. Another possible revolt is when morale is very low, in which case the said planet may split off on its own or join the I-League.