I think this phenomenon is the result of a confirmation bias. Half of all people are below average intelligence, so if you live in a majority democrat region most of the stupid people are going to be democrats the same for majority republican regions. Add to that the social friction with going against the herd, the friction must be worthwhile to the person so it is more likely a lot of thought went into someone's conclusions if they're willing to make their life more difficult for it, ideological minorities like that are (probably not remotely near astronomically) more likely to be a more thoughtful. Both of these pressures skew the majority political leaning in any region below average intelligence and have the opposite effect on the minority.