For a review of a range of evidence and possible explanations, see Henry R. Hyatt and James R. Spletzer (2013), “The Recent Decline in Employment Dynamics (PDF)”, IZA Discussion Paper Series 7231 (Bonn, Germany: Institute for the Study of Labor (IZA), February). These authors suggest that much additional work is needed to understand the role of different factors in changes in labor market dynamism.
For an analysis that raises the possibility that some of these shifts reflect better job matches, see Raven Molloy, Christopher L. Smith, and Abigail K. Wozniak (2014), “Declining Migration within the U.S.: The Role of the Labor Market”, NBER Working Paper Series 20065 (Cambridge, Mass. : National Bureau of Economic Research, April).