From What Does Implied Volatility Skew Measure? - Mixon (2011), it seems that the skew measure you are referring to is inversely related to the spot price (S&P 500 in this case). Take note that the skew measure in Mixon is continuous 30 days TTM (similar to the VIX).
See the figure below for a comparison between Figure 1 in Mixon (2011) and the VIX by CBOE for the duration of Jan 2005 to Jan 2010, which proves that both show a similar pattern and are thus inversely related to the spot (since VIX is well-known to be inverse to the S&P 500 and is used as a hedge for systematic risk ).
You can see that they both spike around the same time below.

