Fat drivers seem to get worse in the summer too.  I think it's because they're too insulated by fat to deal with the heat well, coupled with the fact that they can't wear well ventilated clothing because it'll reveal too much of their disgusting bodies, and on top of that they have to witness fit people dressed in revealing clothing.  It's a combination that seems to trigger their aggression and recklessness on the road.  In the winter, everyone is bundled up, and the blubber of fat people keeps them a bit warmer, so I'm sure they feel much more comfortable and perhaps on equal or greater footing to a fit person.