I don’t know how well validated this is, I’ve only seen the same screenshot multiple times, no others. While if there is a margin call RH would have the right to sell down positions to meet the call, in this case it’s not clear if that’s the reason. It would also be problematic because they and many other brokers first manipulated the market by preventing purchases or the stock, causing it to tank, leading to them selling at a price below what it otherwise would have been.