Yeah, increased black turnout probably was the cause; maybe Reagan's speech arguing strongly for states' rights, which occurred in Mississippi, made a difference.
Looking at the swing map, almost all of western Mississippi swung towards Carter in 1980, and that's the part of the state with the heaviest black population, is it not?
Though it's not entirely regional; some strong Carter swings in eastern MS in counties that were bordered by strong Reagan swinging counties. Probably the black/white divide again, I suppose.
Also look at the exit polls in 1976 and 1980 among blacks.
http://elections.nytimes.com/2008/results/president/national-exit-polls.htmlFord got a surprisingly high 16% among blacks, while Reagan got just 11% in 1980.
