They just come with the station. As long as your station hangar is intact, those garrison units will hyperspace in. Every time the enemy destroys a garrison unit, a new one will hyperspace in. It works just like troops spawning from a barracks on the ground.
The units don't "come from" any particular fleet or system, they come with the station (depending on your station level). Though, of course, you can always pretend they hyperspaced in from a nearby system when they recieved word of the attack.