The fronts are probably doing it too. Given you dry it in the wet and cold, I'm guessing the Brake Rotors are rusting over and sticking to the Brake Pads.

The next time you leave it for a couple of days after getting it wet, look at the Brake Rotors before you get in the car. They will probably be rusted over. They usually do this when you wash the car too. It only takes a few minutes for the rotors to start rusting.

This isn't unusual, and it's nothing to worry about (AS long as its just brake rust)