There's only so much CONTROL or worse, LACK OF CONTROL that a person or society or music can take. (I remember a period of time after the annual Barbequed Baked Beans Competition in which I was the sole judge that I personally do not wish to relive ever again, thank you very much).
By the 1960s composers in general said ''Enough of serialism, already"
or "I'm tired of the 12-Tone method"
or "Remember when music was ENJOYABLE and didn't require a PhD. to listen to it?"
or "etcetera."
And so, two (2) new trends started developing and they were:
1. Quotation Music
and 2. Minimalism (or following the K.I.S.S. principle; i.e. "Keep It Simple, Stupid!")
This time, I shall discuss number one firstly and then secondly, number two.