Winter tornadoes are not as rare as you may think. Anyone with Lilapsophobia (the fear of tornadoes) certainly had something to be afraid of on February 5th and 6th, 2008. A series of massive tornadoes struck at least 5 Southern states (Arkansas, Tennessee, Kentucky, Mississippi and Alabama) causing at least 52 deaths and 150 injuries. Do not forget the children of Oconto, Nebraska who had their Halloween trick-or-treating horribly interrupted by a tornado that swept through the center of town.
No time of year is entirely free from tornado risk. Like spring, fall is a transitional period when masses of warm and cool air are more likely to collide and create the thunderstorms that can trigger twisters. During summer, tornadoes can also form within hurricanes, which can turn a relatively benign outer arm of a hurricane into a locally devastating event.
Winter tornadoes can be particularly deadly, not because they're stronger, but because they tend to move faster.