I wonder if this could be connected to Colin Woodard's argument in his book American Nations (https://www.amazon.com/gp/product/0670022969/) that there are eleven basic regional cultures in North America. According to his map (https://amp.businessinsider.com/images/55b273a2371d2211008b9...), North Carolina sits right at the intersection of three of them -- Tidewater in the north, Deep South in the south, and Greater Appalachia in the west. You'd expect to see lots of linguistic diversity in a place where different cultures are rubbing up against each other.

Interestingly, Woodard's book echoes an earlier work, Joel Garreau's 1981 The Nine Nations of North America (https://www.amazon.com/Nine-Nations-North-America/dp/0380578...). Garreau, however, put all of NC inside his "Dixie" nation, the analogue to Woodard's "Deep South." I wonder how much of this can be chalked up to differences in methodology, and how much to demographic shifts in the three decades separating Woodard's work from Garreau's.

This book

http://www.amazon.com/Nine-Nations-North-America/dp/03805788...

seems to have some connection with the content of this article. The article mentions some cities that are key to their regions. The book breaks up North America into nine regions with economic and cultural connections, and names a "capital" city for each of them. For example, Miami as the "capital" of a region including the Carribean. I think the article mentions some of these same cities as being important centers for their regions.

EDIT: wiki page might be better for facilitating discussion:

http://en.wikipedia.org/wiki/Nine_Nations_of_North_America