This is from the Social Security Advisory Board's Aspects of Disability: Decision Making: Data and Materials.As with many of these charts, it can be read in different ways. One can certainly note the dramatic increase in disability based upon mental illness. However, I would submit that this has far more to do with the dramatic changes wrought in psychiatry by the Diagnostic and Statistical Manual (DSM) than by anything intrinsic to Social Security. If you're not familiar with the DSM, it's probably the biggest development in the history of psychiatry. It's the Bible of psychiatry. While there are plenty of critics of individual portions of the DSM, I think that few people deny that it has been a dramatically positive development. If you are outraged by this increase in the number of people found disabled due to psychiatric illness, be careful to note the dramatic drop in disability benefits awarded due to circulatory disorders -- mostly heart disease. This is also due to developments in medicine, in this case cardiology. You can't take one type of improvement in medicine without taking the other types of improvement in medicine.
My best guesses are that the increase in musculoskeletal approvals has to do with the aging of the population and, perhaps, to different coding at Social Security. I don't think there has actually been much change in Social Security policies or practices in evaluating musculoskeletal disorders.