When someone becomes impaired and unable to work there can be a lot of reasons why. Statistically speaking, the conditions that tend to trigger the largest number of applications for Social Security Disability Insurance stem from physical issues. Whether you happen to be a laborer in the metals industry, construction or a nurse in a clinical setting, physical work is required. It's not unusual for individuals in these lines of work to suffer injuries to their backs, knees and shoulders to a point where they can't do their jobs.
There are some illnesses that qualify for SSDI as well. Some are physical. Others are mental. They can be just as impairing as the more common physical issues, but they are often harder to nail down from an objective medical evidence standpoint.