If the reports I read were accurate, some of the congestion came from jack-knifed trucks. I'd bet the long-haul truckers had no trouble on a mere 2", but the short-haul Georgia drivers may not have seen snow in years. A few slips here and there could tie up a lot of traffic. Car accidents would be a lot more frequent than truck accidents, but they don't usually tie up as many lanes.
I remember what just a couple of accidents did when I was taking some training just north of Atlanta. I'd a crazy notion of driving to dinner somewhere after the class, and discovered that it took an hour and a half to go a quarter of a mile because of people trying to route around the highway. I finally got off in a lumberyard parking lot, walked back to the hotel, walked to dinner, and came back for the car later. In the resulting miles of congestion will be some cars without enough gas to keep the heater running (and no blankets), so some drivers will abandon in the road and walk. If three abreast follow the leader away from their cars, nobody behind will move until the tow trucks come, so they may as well bail also. Crystallization ensues.