We examine risk factors that explain daily changes in aggregate credit default swap (CDS) spreads before, during and after the 2007-2009 financial crisis. Based on the European iTraxx CDS index universe, we document time-variation in the significance of spread determinants. Before and after the crisis, spread changes are mainly determined by stock returns and implied stock market volatility. Global financial variables possess explanatory power during the pre-crisis and the crisis period. Liquidity proxy variables are significantly related to spread changes for financials, while unrelated for non-financials. Examination of the risk factors' explanatory power for large spread changes reveals weakened significance indicating that additional factors are necessary for their explanation. Finally, we examine the lead-lag relationship between spread changes and stock returns. Stock market returns lead spread changes during the crisis period, while a bidirectional relationship emerges after the crisis period. This suggests that aggregate spread changes are actually informative for equity market participants, possibly measuring systemic risk.