Here is a graph showing the correlations between the leading indicators of the economy, Job Openings, Unemployment Rate, and the S&P.
It can be seen from the charts that the Job Openings (.a) historically begin to decline before there is any change in the unemployment rate. A simple explanation for this could be, less jobs, more people unemployed.
Once the unemployment rate begins to creep up (.b); historically this has led to a sell off in the S&P.