Dan Walters in the Sacramento Bee speculates, "A key factor in the potential upset is an extremely low voter turnout." The problem with that theory is that turn-out in the run-off is already higher at 10.5%. Since even lower turn-out in the primary didn't produce skewed results, it's unlikely that low turn-out is solely responsible for these results.
President Obama's approval levels are at their lowest they've ever been. The botched roll out of Obamacare has likely led to depressed Democrats not showing up for the run-off and heavily motivated people voting against a Democrat, similar to what happened in 2010. I'm not going to predict that Obamacare is going to lead to a disastrous 2014 election for the Democrats, but it appears right now it's an anvil weighing them down.