Probably the retiree influx? That fits with SC's swing also being more modest than expected.
There's also the Lumbee areas, which are unique to NC vs. the rest of the South and moved strongly toward Trump.
It also looks like there was some degree of disappointment with Trump in super red Appalachian areas, but I struggle to explain this.
Swings to Biden are pretty strongly correlated with areas heavy with retirees, especially outside of major metros!
The answer is 100% down to the general Biden underperformance in rural parts of the state, which is imo a combo of a swing to Trump among farmers (also observable in the rural midwest), slight swings to Trump among Black and to a lesser extent Latino voters, and the asymptote of rural southern white democrats dying out.
