I have no way of proving this, but my thought has always been that several things happened at about that time (1971) which established what we could call megatrends.

1) Computing: Prior to about that time, only megacompanies could afford computers such as the System/360 and System/370. Anyone else using computing at that time was timesharing off of megacompanies computers, such as Boeing Computer Services.

However, in 1969, IBM released the System 3, which was a system designed FOR smaller businesses. It's footprint grew, and started to have an impact on macroeconomic data in about that 1971 timeframe.

https://www.ibm.com/ibm/history/exhibits/rochester/rochester_4008.html

2) Automation: IBM strikes again: In the late 60's, IBM released numerical control modules for the 1130. These modules allowed the 1130 to control manufacturing machines. So, businesses could start to automate assembly line jobs which previously were done by humans.

https://www.ibm.com/ibm/history/exhibits/1130/1130_intro.html

3) Globalization. This was about the time when companies started to look to move manufacturing operations overseas, either to build closer to their customers, or to drive down the costs of production for domestic markets.

Anyway, those were my thoughts on that topic. In terms of data, then, real average hourly wage peaked in 1974, and things proceeded from there.

To you point, my view is that those who prefer heavy government in the economy to light involvement have been trying to hang the events of the early 70's (or earlier - income inequality bottomed in 1968 and has been rising at about the same slope since) on Reagan's neck as long as I can remember, to the point that whenever I see an economic chart that starts in 1980, it's almost always the case that the creator is being disengenuous.

Know what else started around 1971? Inflation in health and medical care and services started to rise at a higher rate than the CPI.