You can argue it was primarily due to inflation (changes in the economy can never be attributed solely to any one thing).

The upper 0.1% largely owns things that are relatively safe from inflation (like expensive real estate in areas where increases in value have exceeded the rate of inflation for decades) while the lower 50 - 80% does not.

It's effectively impossible to prove definitively, but I find it hard to believe it's a coincidence that the share of wealth held by asset-heavy individuals shot up at the exact same time the money supply increased significantly, especially given that lower class wages were actually increasing faster than inflation and upper class wages at the same time.