I suppose it's an example of the same process by which taboo swear-words lose their power as people use them more freely, so they move on to something stronger. (I won't give examples, because I think we've got to a stage where it's time to go in the opposite direction, so far as swear-words are concerned.)

The word "liberal", after being misused in American politics as a demonising insult word for year in and year out, has lost some its power to wound. Possibly it may even have occurred to some listeners that they live in a country which had always prided itself since its foundation as being a "liberal democracy".

So a totally inappropriate word gets plucked out of the air and used to replace or supplement it. "Socialism" is a word that is identified as a bad thing in the USA - so "Socialist" is the label to apply to Obama, regardless of the fact that there is absolutely nothing "Socialist" about his politics. (The more appropriate term might be "Liberal Conservatism" or possibly "Conservative Liberalism.)

It's rather similar to the way the word "Muslim" has been used - if a word has associations that many Americans don't like, that's the one to use, regardless of the fact that it is totally inappropriate.