Ski,
Don't want to get into a lengthy discussion about how Natural Selection works, but suffice to say, any heritable genetic characteristic that increases a population's lifespan and reproductive success is favored over the long term until most members of the population carry this trait. Heritable genetic traits that reduce lifespan and reproductive success slowly get weeded out of the population, as fewer and fewer surviving offspring are produced with that trait.
For northern deer, rut timing is essential to population survival. Fawns born too early in spring can be killed by late spring snowstorms. Fawns born too late in summer cannot put on enough body weight before winter sets in to survive the brutal conditions of deep snow and extreme cold. Because of these factors, Natural Selection VERY strongly favors deer with a genetic reproductive timing for early November. This produces fawns in late May, protecting them from late cold snaps but early enough to maximize body growth before the following winter.
However, the South doesn't have such a critical time period for breeding/fawning dates. We don't have winters conditions so extreme they directly kill deer. This provides a more "fluid" situation for Natural Selection to work from, and allows small differences in "most productive fawning times" to eventually shift peak breeding times in very localized pockets, based on local conditions. This produces some very bizarre breeding times. For instance, in south Florida, peak breeding is July and August. Why? Because a peak rut in summer produces a fawning time right during the climatological driest time of year (January through March). In the endless standing water of the Everglades, fawns hitting the ground when the swamps are temporarily dry is the difference between life and death for those fawns. In south Texas, water also plays a critical role in rut timing, but from the opposite direction. South Texas is basically a desert. Rainfall and plant green-up is critical for whitetail survival. The early part of hurricane season, June and July, is often the wettest period of the year. The rut is south Texas is in December, which produces fawns right during this hurricane induced green-up. The Hurricane season also plays a role in rut timing along the Carolina coast, but in the opposite direction. Late May and June tropical systems can produce devastating flooding rains along the coast, hence deer along the coast have developed a late September and early October peak rut, so that fawning times fall early enough before the tropical systems arrive that fawns can be up and mobile to escape floodwaters.
The above examples are the extremes, where the biological "reasons" for the unusual rut dates are fairly clear. However, in many parts of the South, especially the Deep South, the reasons for widely varying rut dates across short geographic distances are not so apparent. In fact, some are a mystery. But Natural Selection generally doesn't "make mistakes." As long as those dates persist over long periods of time, there is a reason for them, even if the reason isn't obvious.