Really and truly, I think November and December are some of the deadliest months for severe weather in the Southeast historically, not just recently. I have seen tons of reports and stories about severe weather striking this region in late Autumn and early Winter with far deadlier results than one would typically think of for this time of year.
I believe the spike in severe weather this time of year is, as Dacula said, because of the more potent ingredients (much colder air than Spring but meeting Spring like warmth.) However, I believe the seemingly larger number of casualties is because seasonal association.
People are less aware of the weather this time of year. Even though there have been countless deadly severe weather events in Autumn and Winter, they are so sporadic (rather than the seemingly once or twice a week we tend to get in Spring time) that people just have a hard time associating severe weather with Winter. In fact, I would almost be willing to bet that certain tornadic events that have struck in the Autumn and Winter in our region have been adjusted in people's memories to the point that they may remember them having happened in Spring.
I know my mother has started doing that as she absolutely refuses to admit that they had severe weather during any Winter month before the last ten to twenty years. She swears up and down that it is something that only began recently.
