Answer:
It was a musical invasion. In February the Beatles appeared on the Ed Sullivan Show in prime time in America, and were a sensation. Young Americans immediately started wearing their hair long in "Beatle haircuts". (It was only long in relation to the crew-cut 50s, and nothing like hippie hair). The Rolling Stones released their first album in America that year. Maybe Led Zeppelin too. Possibly also the Yardbirds, with Eric Clapton.