Whitetail does that don't conceive on their first estrus cycle re-cycle about 28 days later. The number of those does varies year to year and herd to herd — heavily hunted herds with skewed sex ratios can have meaningful second-rut activity because not enough bucks were available to cover all does in the first cycle. Healthy, well-balanced herds have a much smaller second pulse.
Doe fawns also play a role. In good-nutrition years, female fawns born the previous spring can reach the ~70 lb weight threshold to enter estrus, contributing to a small but real December breeding event. This is more common in agricultural-belt regions than in big-woods country.
What second rut looks like on stand: short bursts of cruising and chasing on cold-front days in early-to-mid December, often clustered around food sources where doe groups have stabilized for the late season. It's spotty — some areas see clear second-rut sign, others see almost none.
The most reliable way to plan around it is to check your state's harvest data and breeding-date studies — see our state rut date pages for the late-season pulse where the data shows it's worth hunting.