Republican Congressional candidates may also drive turnout. During the redistricting process, the North Carolina legislature drew some of the state's rural Democratic incumbents, such as Democratic Reps. Heath Shuler and Mike McIntyre, into heavily-GOP territory, which has encouraged multiple Republicans to enter the races. Their districts are much more conservative now.
At the same time, Democrats and supporters of same-sex marriage do not have enough strongly competitive primaries of their own to incidentally drive out votes against the measure. That gives social conservatives an immediate leg-up in voter turnout.
The issue of same-sex marriage was originally supposed to go before voters on the same day as the general election in 2012. However, one of the Democrats voting in favor of the amendment said that he did not want it to be a political item, or a wedge issue to drive out Republican votes against Obama and Perdue.
Thus, the May election is, in a sense, a compromise. Though Obama carried the Tar Heel state in 2008, the state is still more culturally conservative than many others.
Given the likelihood of the amendment passing regardless of timing, conservative Democrats decided not to gamble by putting the issue up to the voters in the general election. In essence, if they're going to lose the vote, they might as well get it over with early than have its coattails hurt their candidates.