UC's are a temporary free occupation as they have a stability rating in %. This rating drops 3% a day, when it hits 0% the UC disappears. Approximately once every fortnight the UC spawns ships to attack the occupier. This fleet needs to be destroyed before the player can pillage the base. If the occupier has limited ships holding the base, the UC fleet can attack and take the base back.
UC bases actually generate fleet, if they are occupied, and attack the occupation fleet. This is called "spawning", and it happens on a predicable schedule. The spawned fleets are never very large, so it is simply a matter of keeping a large enough fleet in place to survive the attack.