What caused the Sixties?

The great mystery of my lifetime has been the 1960s. It’s worth returning to this vast subject periodically as new perspectives unveil themselves. 

The closest thing to a successful prophecy of that era was made by science-fiction writer Robert A. Heinlein in his 1941 Future History chart, in which he foresaw the coming “Crazy Years” when there would be a “gradual deterioration of mores, orientation and social institutions, terminating in mass psychoses….” (By the late 60s, hippies were so drug-addled that they made Heinlein’s self-indulgent 1961 book Stranger in a Strange Land into a cult novel.)