Commenter mtltechwriter has another theory: "the changing position of earth's magnetic core." And he blames inattention and those crafty Germans.
This has been happening almost unnoticed because most scientists were busy with other projects. The recent increase of teutonic activity at the earth's crust however, has shifted the earth's balance and therefore the moon appears to be moving away.
Well, achtung, Fritz: you might be able to pull the wool over Frenchie's eyes, but America's not going to stand for this.