I've been debating the subject on the AH forums; I say due to racist sentiments and a lack of black voters to offset white Southerners going from Kerry to McCain, while someone else says that they're just naturally trending Republican.
Same thing, in a sense.
Though an important part of the equation is that they have relatively high numbers of Southern country folks who still voted Democratic as late as 2004 - it may sound perverse, but part of the answer is comparative weakness (but
not absence) of racism as a determinant of voting patterns.