utvolsfan77,
Without question, some late fawning is due to female fawns achieving the necessary body weight to enter estrus when they were a fawn. But some individual does just have a very late personal timing (which appears to be genetic, in that it occurs in that same doe every year).
And that is how very localized breeding timings develop. Breeding timing is a bell-shaped curve, and if an earlier or a later than normal timing is advantageous, then those on the outer legs of the bell curve do better and pass their personal timing down to their offspring, which eventually skews the entire bell-curve earlier or later. And it really doesn't take long for this to occur.