It would be a high demand for the # of cars that they have, remember that at this time FL cars are migrating to other places because the travel season is slowing down for FL (summer is over)
No, actually the one-way deals to leave FL happen in the late spring shoulder season, when the FL winter/spring break high demand period is over. The summer in south FL is off-peak, due to hurricane season and nice weather up north.
Oct-Nov is another shoulder season, but in reverse. The northern markets cool off, and demand increases in warmer southern markets. So the rates in FL tend to creep up, and sometimes one-way deals from the Northeast are offered. Hence the high rates the OP saw in Oct.