The first major upsetter of the newly defined applecart was, of course, the first wave of rock'n'roll. Fuelled by high-powered radio stations in the American South flooding the nighttime airwaves with rhythm and blues, the music wasn't supposed to go to a white audience, but did.
But with Elvis Presley in the Army, Chuck Berry in jail, Little Richard getting religion again and Fats Domino mining an inoffensive line of R&B, the innovators were out of the way. It meant the imitators could take over in the States. The rise of skiffle in the UK was a separate and intriguing development that brought its own, separate legacy.
As things settled down, there were some interesting developments in the late fifties and, particularly, in the early sixties. The R&B influence permeated the popular airwaves, but the emerging conventional wisdom had a clear distinction between performers and the people who delivered the musical material and made the records.
Artists, needed songs to record. Writers produced the material. Producers (more accurately, though they never got the recognition, engineers) who made the records and disc jockeys who helped determine what actually sold.
And when we look at the sixties, we're looking at the blurring of lines between those newly defined roles. The major significance of The Beatles was to break the line between writers and performers. And, increasingly, as the artists started to get their heads around the sonic possibilities, thee's a blurring of lines between artists and producers, and between producers and engineers.
So a whole string of factors aligned themselves between 1963 and the end of 1968.
A Flurry of Post-Beatles Experimentation The Response Across The Pond Folkies and Folk-Rockers
East and West Coasters San Francisco The Tide Recedes (And Flows Back In)