I'm not an economist but I do wonder if the reasons some of those places have higher mobility is because the previous generation in those areas have relatively low income jobs (i.e. mostly agriculture) and many of the current generation have a "gotta get out of here" mentality leading them to better paying opportunities elsewhere. I know I felt that way living in the mid-west in early high school.

I'd like to see a map of how many of these income reporters moved away from their locales and to where and also what professions they picked up.