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.