After substantial growth in the 1990s and 2000s, enrollment in the U.S. Social Security Disability Insurance (SSDI) program has been declining since 2013. We use detailed administrative data to quantify the contributions of various factors to trends in SSDI enrollment, focusing especially on the decline in the 2010s. A statistical decomposition suggests that the vast majority of the decline in SSDI enrollment since 2013 is attributable to declines in application rates -- and, to a lesser extent, award rates -- within demographic groups. There is very little contribution from changes over time in demographic characteristics, eligibility, or exit from SSDI. The decline in SSDI enrollment rates is disproportionately driven by older low-to-middle-skilled men with relatively severe health conditions who, over time, have become less likely to apply for SSDI and more likely to work. Consistent with this descriptive evidence, we present results from a causal analysis suggesting that improved labor market opportunity for less-skilled workers is a key explanation of the decline in SSDI enrollment. We also investigate several other popular hypotheses for the decline in SSDI applications, including lower award rates at the appeals level, and find evidence at odds with them.