First, it's right in line with the Republican Party meme of Obama as the alien, sinister, Kenyan, worldly, U.N.-loving "other", who doesn't look like us & doesn't "share our values". It's a transparent play to the fears & prejudices of white Americans, specifically the less educated among us. Just look at the "wink-wink" nod the party has given to the "Birther" movement. On the whole this isn't a terribly sophisticated group that they're playing to, but the fact is that they constitute (by my estimate) somewhere around 25-30% of the American electorate. Scary.
But there's something else at work here too. By labeling Obama's essentially centrist policies as "socialist", it effectively defines those policies as politically out-of-bounds, as unacceptably leftist. Rather than a pendulum, use the metaphor of a spotlight that swings back & forth from time to time. What's within the spotlight at a certain time is what's considered acceptable to say, or considered within the bounds of the politically possible. Since the very word "socialist" has long been demonized in American political discourse, labeling his policies as such helps to move that spotlight further to the right. Thus, even things like modest health-care reform, economic stimulus & expansionary monetary policy are labeled as politically unacceptable.