Speculation that Robinhood was limiting the trading ability of those users at the behest of, pick your poison, Citadel, the US government, hedge funds, Janet Yellen, or others, ran rampant.
But none of it was true – at least according to Robinhood's telling. In its post, Robinhood wrote that (emphasis TechCrunch):
[a]mid this week’s extraordinary circumstances in the market, we made a tough decision today to temporarily limit buying for certain securities. As a brokerage firm, we have many financial requirements, including SEC net capital obligations and clearinghouse deposits. Some of these requirements fluctuate based on volatility in the markets and can be substantial in the current environment. These requirements exist to protect investors and the markets and we take our responsibilities to comply with them seriously, including through the measures we have taken today.
That reads like Robinhood ran low on capital and had to make some hard decisions, quickly. The securities its users wanted to trade likely generated the highest capital obligations given how volatile they proved and how long it takes for trades to settle, so Robinhood had to shut off some trades to stay on the right side of its capital needs. (Not great, not terrible?)
Reporting from Bloomberg indicates that Robinhood "tapped at least several hundred million dollars" from credit lines today makes sense in this context. As does the unicorn's decision to allow for some trading of the afore-limited securities in the near future ("starting tomorrow, we plan to allow limited buys of these securities," the company wrote); now reloaded with more capital, Robinhood can afford to let its users get back, somewhat, to business.