Quite frequently, when doing and air-layer, the bottom part will begin to flush new growth. If the bark is girdled (removed) as part of the air layering process, the lower portion of the plant, or branch, essentially does not see the part being air-layered as being there any more because it receives no products of photosynthesis. So it pushes new growth from the buds that now have apical dominance (upper most) in the same way that it would do if you had just cut the branch off. So while your air-layer is rooting, your original plant is getting on with the business of regenerating growth, as well.