As it has been explained to me, there is a vein that feeds the antler base, even after it hardens. At a certain point during the year (different for each animal by small margins) if the temperature dops below freezing the vein dies off, and the antler comes loose from the base. In warmer climates, the new growth chokes off the vein, causing the antler to drop. I've seen deer missing antlers in late December, and I've seen deer that still had both until early March. I'm sure that health, and diet have something to do with it as well, but thats more biology than I care to look into.
gd