People reacted to the sheer disaster called the Bush-Cheney presidency even if they were center-rightists, and the only reason I think is the most convincing as to why the 2010 midterms turned out so bad for Democrats is because of the fear of Obama as a foreign agent and, for the racists, because he was of the wrong skin color. They energized their base that year painting him as an extreme far left Muslim president.
I don't think the pendulum swung that far between left and right, though. The real swing of the pendulum occurred after the re-alignment of both parties following the 1960s. It became a choice between centrism and extreme right-wing nuttery. In that respect, of course Obama was the natural choice of the left compared to McCain-Palin. Nobody wanted George W. Bush Redux.