They fight for bird seed. Notice the tail covers the genitals of the second buck. It appeared to be that the aggressor was going straight for where it hurts. Even though the Key Deer look very docile it is a cutthroat world of survival of the fittest.
There were 8 bucks all in velvet at the Hang-Out. The antlers become covered in a soft fuzz resembling velvet and then peels off when completely grown. Every successive year the antlers grow larger.