I have heard that in poor economic times people move to areas that cost less in heating and where they can survive the summer with less cost.  This may play somewhat into the moves.  While that is one theory, there are some facts that make the south attractive.  Gasoline is usually cheaper in southern states.  Florida and Texas have the current highest rate of job growth and no income tax so that could help explain their rankings.  In fact, southern and south west rank pretty high as retirement places.  All in all, I can see reasons for the trends.