There is speculation - though I have no idea how founded it is - that major hedge funds pay Robinhood for advance notification on trades, which allows them to theoretically undercut the trades and make a fractional profit.
Say, for example, you put in an order to buy 5 shares of Gamestop stock at market price, on Robinhood. Citadel has access to this information, because they pay Robinhood for that access. Market price is increasing from, say $1.00/share to $1.01/share. Citadel sees this, and undercuts the trade, buying 5 shares from the market at $1.00, and selling them to you for $1.01. You, the consumer, just see that your market price order ended up costing $1.01/share. Whereas Citadel skimmed some profit off of it that you didn't see.
Citadel and other hedge funds are taking an absolute beating from this, because they've bet very, very heavily on companies like Gamestop, Blackberry, Nokia, and others to fail. I saw an estimate this morning that hedge funds had lost approx $71 billion year-to-date on this fiasco.
So Citadel gets in front of Robinhood and demands that Robinhood stop letting people buy these shares. That depresses the price, causes a bunch of holders to try to sell to lock in whatever they can, and allows Citadel and other companies to cover their shorts. Basically, the big hedge funds pressure Robinhood into stopping new purchases of these companies, because without Citadel paying for those services, Robinhood doesn't exist. Robinhood caved because the other option is turn off the money tap that funds their business.
This is the rampant speculation, at least.