Jackson Costa has done a study, published in the Social Security Bulletin, on the decline in earnings prior to claims for Disability Insurance Benefits. Below is a chart from the study.
The study demonstrates something that's obvious at ground level -- for most disabled people, disability isn't something that happens all at once. It comes on over the course of years.
This is important because those in Congress and the higher reaches of Social Security tend to visualize disability as mostly associated with trauma but that's wrong. Trauma is actually a relatively minor source of disability. It's illnesses that accumulate and worsen over time. Often it's more than one thing that disables a person. People try hard to fight off disability. Often they wait a considerable period of time after stopping work altogether before filing a claim. People don't like to have to concede that they're disabled.