This is most likely the answer. They are taking on a lot of risk allowing this type of trading behavior. They likely don't have suitable risk infrastructure developed to feel like they have a handle on it, so they shut it down.
Additionally, other retail brokers selling to Citadel Capital have done the exact same (Schwab comes to mind, but not only).
Other brokers however (fidelity), have not. If the short squeeze is to happen tomorrow, this seems an unlikely coincidence that those retail brokers affiliated with Citadel Capital tool positions that would ultimately deflate the stock.
I don't see how GP can assure that no short sellers is behind those moves.
FINRA arbitration, and the FINRA complaint process, is highly sympathetic to retail clients. When these clients lose money on GME et al, there will almost certainly be a class-action lawsuit for some fraction of their collective losses. And Robinhood's lawyers will almost certainly recommend they settle. That is the risk, beyond margin lending and options settlement, they are seeking to mitigate.
(Also, when that money is lost, there is a decent chance Robinhood will be fined by half the regulators on this planet for inducing people to overtrade through its gameified interface or something like that.)
If their customers - the firms that pay them, not app users - see the RH platform as a threat to their business they could pull the plug[0]. Or if this prompts regulators to examine RH and similar products.
The risk isn't in the outcomes of options/trades; its risk to their business model.
[0] RH's customer are actually market makers, who by and large will be profiting heavily off of this.
This point has been made repeatedly elsewhere but it bears repeating. Market makers are getting rich off this trading. Robinhood's customers are not hurting from this.
Still, as far as risk to RH is concerned - if this kicks off a change via their customers, regulators, or legal action the change is probably not in their favor.
On one hand, you could say RH shouldn't let people buy options if RH doesn't believe those people can pay up when the option expires. On the other hand, you have people buying options who maybe don't understand that you need cash to buy the shares if the option expires in the money.
There could is also the concern about new accounts trading in GME with funds that might be suspect. It's a risk you don't want to take if you don't know your customers and the bubble is ready to burst at any moment.
Interactive Brokers hiked the long margin to 100% (and short to 300%) which is a reasonable way for a broker to behave.
Killing the ability to buy is not.
The answer is no.
No, people / pundits / the media were widely proclaiming that the healthcare system would be overrun and the next great economic depression was upon us. Most people assumed the stock market would continue to fall.