There are many factors that could contribute to a doe not having fawns, each of which may or may not play a part in when she comes into heat. I don't have any articles or research to back me up, this is just from my own knowledge of deer I've picked up over the years. Deer, like humans, are individuals and each one is a little different. Although deer as a species first go into heat around the same time regionally each calendar year, an individual deer may be weeks ahead or behind that schedule for various reasons. I do believe an individual deer will go into heat around the same time each year, so what may be happening is those does you're seeing being tended naturally go into heat earlier. This could lead to them being bred and consequently giving birth earlier. If the whole process happens too early, and the fawns are born before the habitat has had a chance to recover from winter, they may stand a poorer chance of survival. This could be one reason those does don't have fawns. It makes sense on an evolutionary scale that those deer wouldn't be allowed to pass on their genes because they don't do the species any good. I have to imagine the production of milk has a great deal to do with hormones, and if a doe suddenly loses the need to produce milk, say if her fawns are no longer with her for whatever reason, her hormones may shift. I have no idea what role the hormones play in what time a doe goes into heat, but one would guess there's some correlation.

Like I said, I have no facts to back any of this up, but it is something interesting to think about. I believe the understanding of deer biology will greatly increase in the next 10-20 years and hunting will continue to change at a rapid pace. For better or worse, we shall see.