If Citadel is a client of Robinhood, then they are are under no obligation to buy Robinhood's product. It's possible that Citadel stopped buying the order flow from Robinhood because they had exposure to clients who in turn had short exposure to the stocks in question. Even in the absence of such an ulterior motive, Citadel is a market maker and it would be foolish for any market maker to do huge amounts of market making on a stock that it's the headlines and is unpredictably skyrocketing. Ultimately Robinhood cannot continue to offer free orders on a stock if Robinhood's clients are not willing to buy a order flow on a specific stock from a specific client because that client's order flow is toxic and making the stock too volatile to make an orderly market in. Declining to accept toxic order flows and withdrawing liquidity from volatile stocks is a reasonable and accepted practice for market makers. Robinhood users pay zero dollars in commissions, and they get what they pay for.