Autogen is suppressed automatically by the sim when we need to create an exclusion zone to remove any possibly conflicting objects from other sceneries or from the default scenery.
How it extends *outside* the airport property, depends both how the exclusion zones are made and how the airport is generally shaped. Ideally, it would be less visible if the airport perimeter was exactly squared or rectangular and perfectly north/south or east/west oriented, because that's the only possible shape of the exclude zone: they can't be rotated or have being of a polygonal shape, that's why autogen suppression doesn't always matches the airport perimeter.