Turns out the jury is still out on this one, but the prevailing thought is that they are marking their territory, since it is mostly intact males who exhibit this behavior, and sometimes neutered males or females.
What works for us to curb the scratching (so we don't get grass in the face when picking up the poopoos), is to break the dog's concentration after pooping with a "here we go" and gentle tug on the leash.