I'd imagine the native civilians and citizens living on the surface,along with the factional citizens living in atmospheric cities,would be disgruntled with the ruling faction because of propaganda would, when the Planet Allegiance gets very low(like about 30-20%), they would slowly start attacking low level Government facilities like post offices,banks,etc then it would slowly progress up to medium and high level Governments facilities and then, finally, the dissidents rebellious,terrorististic actions would get to the point that the Aliens would get so fed up with it,that they would just say"screw it!" and abandon the colony.