I think the squishy (hey! I have a new favorite word tonight!) swing voters just turned against the Republicans en-masse.
Also, they eased their conscience in a lot of red states by either believing (or forcing their conscience to believe) that they voted for a Conservative Democrat.
Not many of these democrats ran as a real Democrat. But more like Reagan Democrats.
Thomas Sowell wrote on article about that "A New Voter Fraud: Democrats are getting people to vote for moderates in order to put extremists in power.". It's on the National Review website.
http://article.nationalreview.com/?q=ZGI1YjRkZTU3ZDIzZDM4OTkwMzUxNTMzODBiNTViM2U=