I can't get past the paywall, so maybe the article answers this question. But, in case not, one major reason could be margin. If you pay Robinhood $5/month then

I can't get past the paywall, so maybe the article answers this question. But, in case not, one major reason could be margin. If you pay Robinhood $5/month then you have access to borrow money from them to buy stock. The first $1000 is interest free and additional "margin" incurs interest. When the stock market plunges, some people will see that as an opportunity to buy more stock at a reduced price. If they don't have any liquid funds in their account, borrowing money from the broker is instantaneous (assuming you've previously enabled it). I don't know what happened, but I could see their line of credit getting eaten up very quickly if enough investors decided to leverage their available margin. If investors sold those shares within two weeks, they could pay back the credit line.