> do most investors really take out margin loans to buy stocks?

In the scenario I describe, those investors take margin loans to long or short crypto, and when they fail, they sell other assets to cover themselves.

This is just one possibility I describe.

> but i'd imagine it's not a majority.

No one knows. It may take only one institutional investor who lost a gigantic amount of money in crypto trading and were therefore forced to sell stocks to shake the stock market.