For many reasons (including the ones above) it's difficult for any institution within reach of the US government to analyze how the alt-right took power but from what I can tell the US economy is in a slow burn. It's been receiving patches roughly once a presidency but it turns out you can't combine a lot of short term solutions to make a long term one. Fixing the economy would require bold decisions and the parties took two different directions. The Republican party realized that any bold policy would get votes regardless of any other factor including coherency. This is why Trump supporters, when asked about their logic usually give some form of "things are bad, and they didn't used to be".
To summarize, there are competing ideas for what got us here, but I think it was less of a real inciting event like WW1 and more of a breaking point that was eventually reached.
To summarize, there are competing ideas for what got us here, but I think it was less of a real inciting event like WW1 and more of a breaking point that was eventually reached.