The first snowstorm is usually pretty bad. Lot's of people lose their inclement weather driving skills. They are going too fast, and not giving proper following distance. It's pretty simple, really. You have to factor in people who think their vehicle will protect them from everything with sensors and anti-lock brakes, and all the other features and gadgets.
And, then, you have the straight up asshats, morons, CDL drivers with 3 months experience, and people too clueless who just should have stayed at home.