"In the South, there's just more folks who need killing."

[morethanpretty]

or example, an associate of the experimenter would "accidentally" bump into the subject while walking down the hall and mutter "asshole"

Maybe the southern had a stronger reaction because this is a very rude thing to do. If the person had said "I'm sorry" or "Excuse me" I bet the southern's reaction would be very different. We take good manners very seriously. Although, unfortunately, that seems to have become less extreme.
I realize this is only one example, but if the other experiments were the same, then its probably is the perceived lack of manners/rudeness that is causing the high number of "violent reactions" on the southern side.