Another possibility (my guess, no research to support it) is that we really have experienced a reduction in productivity growth. Corporations are doing very well post-recession, labor is not. I say and think this over and over again: corporations have figured out how to take a larger share from productivity than labor has.
At some point, labor has to wonder "why should I?"
Skip to about 1:00 if you like. https://www.youtube.com/watch?v=_iiOEQOtBlQ