The answer to your question is testosterone. An increase in testosterone in late summer is what causes a bucks antlers to harden and shed velvet. A decrease in testosterone in late winter is what causes the bucks antlers to fall off. This begs the question.....why are your bucks holding their antlers later? The most likely answer is your buck to doe ratio. When the ratio is heavily skewed in favor of does it can take two or three heat cycles before all of the does are bred. When this happens, bucks maintain higher testerone levels later in the year.