The main reason for the decline is that better economic opportunities have attracted workers who had filed for disability after losing jobs. According to the Bureau of Labor Statistics, the number of disabled in the workforce hit a trough of 4.5 million in January 2014 but has since grown by 1.5 million. The labor-force participation rate for disabled working-age men has climbed to 38% from 30%. This suggests that for many Americans disability had become another form of unemployment insurance.
But for the report as a whole see "Analysis of the 2019 Social Security Trustees’ Report".