Although Pew doesn’t provide us with much data about why these changes are taking place, I think several factors are at work. First, there are more unionized workers in the public sector now than in private enterprise, and dislike of the arrogant, incompetent DMV worker who cannot be fired and gets a better pension than you do runs pretty strong.
Second, more people think unions won’t work in the jobs that they have. If costs go up, business shifts to non-unionized firms either inside or outside the US. Third, the perception that bailouts have gone to sustain wages for unionized workers has not resonated well with people who think they will pay taxes in order to support higher wages and benefits for other people. Fourth, scandals and overreaching (ACORN, card check) have sharpened public distrust of the labor movement.
Pivoting of an NYT article, Sonny Bunch criticizes NYC's teachers union.