First of all keep in mind that if you don't know the identity of your grandfather, you don't really know "all" of your ancestry. You have no way of knowing where his father and mother, his grandparents, etc. would have lived so it is entirely possible that at least some of them might have been in the South at some point and thus had the opportunity to spread branches of the tree, so to speak and create lines of descendants who are today showing up as your DNA matches.

In my own case all of my known ancestors have lived nowhere but in the South but that doesn't keep me from having lots of DNA matches with people either living in other parts of the country today or whose ancestors have lived up North for several generations. Also keep in mind that just because there was a relatively recent non-parental event in your tree, that doesn't mean that there might not have been other such occurrences in the past. There could have been an adoption, official or otherwise, infidelity or who knows what.

I took a Y-DNA test and ended up having no matches who shared my surname; the genetic distance with my matches, all of whom share the surname of Short, however, is such that the non-parental event probably occurred many generations ago, maybe even back in Europe. There have presumably many generations of my family who have never known that they are actually members of the Short family, genetically speaking, rather than the Elkins family. So, as you can see, the reasons for you apparently having Southern cousins could be many.

Michael