From my limited understanding, I think the general financial system with the banks lending large amounts of money and people accepting it because of very low interest rates contributes massively. There is a lot of extra cash floating around with unemployment checks, stimulus checks, tax credits, as well as easy credit, which means that mortgages increase, people's home value increases, they sell and push it up. Then people are spending more money on stocks, which means stock values go up, people believe they are richer because of the paper value of stocks, homes, etc. There has been a large amount of hype around all of these, eventually it comes crashing down and someone is left holding the bag with whatever position they were in.

You forget upper middle class saved a bunch of money during lockdown (no travel, no dining out, no commuter expenses etc), along with having IPOs or other investments increase in value.

It's not that money is abundant, it's that debt is cheap.