I would wager there are some people upset that he won the election and are grasping for anything in some attempt to make his victory illegitimate. I would guess that group is the minority however.
Everyone I have spoken to that questions where he was born does so on the grounds that the precedent established by not showing he was born here conclusively is in effect kicking the Constitution out the window.
Now, I am of the opinion that the issue has already been proven and ought to be put to rest, but I think the notion that a majority of people continue the issue on the ground of race is ridiculous.
Further, I think this poll does not really show much of anything because from the link it is impossible to determine how many of those in the South questioning his birth are Republicans, Democrats, or other. While many of the Southern states did go red, the President got millions of votes down there as well. Further, I want to see how the regions were defined.