IIRC, JFK got the ball rolling during his administration.
The late 60s was the beginning of the “only the mad are truly sane” movement, which was actually an offshoot of Langian radical-leftist psychological theory. Lang blamed everything on capitalism.
This bled out into the media hyperspace, and suddenly we got bizarre reports such as Meadobrook, meant to rev up public animosity towards mental health institutions. Meadowbrook was actually a good institution, and since I was working in something associated with the mental health field at that time, I knew that patients at most NY mental health hospitals were treated very humanely and that many new programs had been instituted to reflect new thought on how to deal with the mentally ill.
But as usual, the media got hold of it and ran with it because it supported the Dem mantra that our society was sick because of capitalism and the mentally ill were just mirroring this, and therefore hospitalizing them was political repression. (Tell that to somebody who’s hallucinating.)
Unfortunately, it hit a bunch of state-level short-sighted fiscal conservatives who thought they would save money by deinstitutionalizing these people and putting them all on welfare or getting them on SSI.
This was a double whammy that destroyed the mental health systems of all of the states and led to untold misery and huge expenses because of the problems created by the free-roaming mentally ill.