There was a study which I cannot locate right now which discussed that one of the substantial impacts of a Walmart moving into a town was that all the "community leadership" type jobs left. Things like accountants and office admins and other business professionals. This was because the small businesses Walmart replaced were the ones that used these services locally. Walmart did these things at it's corporate office far away from the community.

So those who did mid-level things like accounting or office administrative work could no longer find work locally and either had to move or take a lower paying service worker position at Walmart.

Essentially what that reveals is that most of these mid-level jobs are susceptible to the Balassa-Samuelson (BS) effect [1]

Which means that most jobs that do not require physical labor onsite are either being rapidly outsourced to cheaper markets or automated if possible. This ends up being that group in the middle of the pay range and likely accounts for the reduction in overall wages for that group.

[1] http://www.bankpedia.org/index.php/en/87-english/b/23137-bal...