Bone Addict nailed it. Hormonal changes brought on by the length of the day. Antler shedding is linked to decreasing testosterone levels. As testosterone levels decrease, special cells called osteoclasts form at the juncture of the skull and antler. Osteoclasts reabsorb calcium from the antler, weakening the joint between the skull and antler until the antler drops. As with anything hormonal, individual animals may release the hormone earlier or later than others. That's why you see some early on in the year that have lost their antlers and some very late in the winter that still have them.
Some species hang on to them longer than others also. Part of it could be when the breeding season is, or if a "second rut" kicks in. Or even how far north the animal lives. Biologically there are reasons that packing a round a large set of antlers could be bad. Carrying that extra weight on a moose or an elk would mean that much extra energy would be expended during the winter when calories are harder to come by. Not to mention the males have already spent most of their fat reserves fighting for mates. The farther north you go and snow increases it becomes even more critical. The only use the antlers are after the rut is possibly as a weapon to fight off predation. But there could be a trade off there as far as the animal getting back into good shape.
