Wretchard said:
“The answer is probably that the voter’s preferences were very susceptible to an alternative orientation. Given an alternative which produced a lower-energy solution to their preferences the same sets of voters could realign very quickly along another axis. Once Scott Brown became a credible candidate with a credible message, one which aligned with the voter preferences in a more natural way than Martha Coakley’s then the stage was set for a rapid realignment.”

I like this analysis (Wretchard, you’re the greatest!). It’s a nonlinear system that can support many different solutions for a single set of boundary conditions. It only takes a tiny perturbation to kick the system from a meta-stable solution to a more stable solution. Nature prefers the solution that requires the least amount of energy and/or coexists with the greatest amount of entropy.