1. From somewhere other than your property and they find your foliage nice and appetizing.
Or 2. You have more than just a swarm. This can happen if there is an existing hive that is in your wall, or somewhere on your property that just decided to swarm and now that you paid to have the swarm removed, you noticed that there are more bees hanging around afterward.
Either way you should have the swarm removed to prevent them from taking residence somewhere (or somewhere in addition to the existing hive)