There's an excellent discussion of this in one of the definitive books about cultural color usage, "Pantone: The 20th Century in Color". Pantone, for those unfamiliar with the company, is the long-time publisher of the de facto standard for color management and description in the print industry, the Pantone Matching System. In addition to PMS, over the last few decades, they have become a definitive source of information about the cultural cues and consequences that relate to color use. (I don't mean to sound like a fanboy...I ran a design agency for 5 years, and they were virtually inescapable when dealing with print).

In the book, they specifically address the color palette you mentioned. In particular, they discuss the origins of the earthy colors during that period, Harvest Gold, Burnt Orange, Tortoise Shell and Avocado. That's a link to the book snippet available through Google Books, so if it doesn't work properly, one can still reference the original source by searching Google Books.

In short, after the 60s, consumers embraced styles reflecting earthy tones, seeing them as "sincere", "stylish" and contemporary. They were a response to the more psychedelic and corporate color patterns that had come before them.