It's down to minutiae in the collateral brokers have to post to ensure that the clearing house doesn't eat losses. Trades on Wednesday aren't settled until Frid

It's down to minutiae in the collateral brokers have to post to ensure that the clearing house doesn't eat losses. Trades on Wednesday aren't settled until Friday, so if a brokerage firm dies in the interim they have an obligation to complete a bunch of trades that they might not have cash for, and unwinding that exposes counterparties to the price moves in the interim. RH users have been buying a lot of meme stocks that move a lot in price, so they have to put up vast amounts of funds to ensure that nobody else suffers losses if they were to back out of the unsettled trades.