Would love to hear a detailed explanation, but my guess is that the deep recession of the 1970's forced companies to look at new ways to cut costs. That was the beginning of offshoring, ending pensions, and employee turnover. The recession gave cause and excuse to make the cuts, and companies found out they never had to go back. Nor could they necessarily have, given that they were no longer competing with a bombed out world following WW2.
* End of the Vietnam war. Deep reductions in the draft released many young men into the workforce and allowed them to start businesses. During the war, only being an exempt student or employed in an essential job could keep you out of the way of bullets.
* The beginning of mass immigration. After the long pause between the 1924 immigration act and the 1965 act, immigration ramped up again. Mass Latin American immigration stepped up -- documented and undocumented -- in the 1970s and continued rising. Dozens of work visa programs brought in Asian and European workers.
* End of the gold exchange standard in 1971. Gold didn't actually circulate but was a synecdoche for the Bretton Woods system of fixed exchange rates. Bretton Woods kept world currencies stable and made trade very hard to sustain because currency values couldn't adjust to compensate for imbalances. Soon the European currencies would float entirely free and world trade would be able to take advantage of differential wages more easily.
* China opened. Mao Tse-Tung was getting senile by 1970 and progressives in China opened up the economy under his nose. By the late 1970s, radical leftist Deng Hsiao-Ping had seized control of the country and implemented the wholesale neoliberal reforms that have turned China from a land of North Korean and Mauritanian backwardness into a modern wealthy nation leading the world. Competition from China could be affecting wages all over.
* The first Mideast oil crisis was in 1973. The USA reached peak oil production in 1970 and started to decline, though Prudhoe Bay created a brief recovery later on.