I gave the explanation I found in the post above. It is moving rapidly though the interstallar medium. It isn't that the expansion didn't start out even but that it is being pushed back by its own high speed motion. This is why it is so bright on that side. The bow shock energy increases its emission intensity besides the central star being closer to that side. Space isn't empty. Give a bubble enough speed through it and the interstellar gases will push back.

Rick