I have a dark suspicion that it has something to do with the rise of computers, of information economy. When some people can take advantage of computers and others can't that leads to income disparity.
The information economy has had a big effect - a silly amount of power is concentrated in US software houses. But it is more plausible that the major trends are slow financial decay as markets get further divorced from reality and truth. Tech has resulted in massive productivity gains; but productivity gains previously resulted in pay increases.
I hazard that sensible investment isn't being rewarded and wages are decaying because of that. In a world where WeWork and Uber are considered great ideas someone who is investing in boring-and-steady infrastructure or basic food & comfort services is not going to get the capital allocations they deserve. That is finance and politics, not tech.
Also the site thinks the answer is moving off the gold standard - I like the gold standard, but moving off it was probably a symptom and not a cause.
For me the defining thing is the emergence of the USA from the postwar politics and policies and into more of an assumed administrator of the globalization project, a status that it retained until just recently.
Besides the information systems, international containerization standards were settled around this time and jet travel was becoming more affordable. A lot of pieces were in place to renegotiate trade and labor deals. And there was a break with numerous social policies; the calls from think-tanks to reinstate a stratified society started coming out at this time too. Big public infrastructure projects mostly died out in the 70's, and the narcotic drug trade from Mexico and the resulting "drug wars" picked up in the late 60's, conveniently just in time for amphetamenes to fall out of favor.
So, the move away from gold is just one of numerous things that changed in a surpisingly short time frame.