See [1].
Long story short: Robinhood lets you buy and sell a share instantly. In the real world, those trades take days to settle and clear. To bridge the gap, Robinhood loans you the difference. And Robinhood, in turn, is "loaned" [2] some of that difference by the DTCC.
So when the cost of that loan goes up, their costs go up, and they responded to that cost pressure by turning off trading. (There are other options.)
[1] https://news.ycombinator.com/item?id=25950361
[2] In quotes because it's technically a collateral requirement. If you sell $100 of stock, DTCC may tell Robinhood it only needs to put up $2 of stock as collateral while Robinhood gets the rest from wherever it's getting it from so it can pass it along to the next person. The $98 is "loaned," as in it's owed to DTCC. If Robinhood fell down, DTCC would sell that collateral and buy the rest of the shares in the market. In this case, DTCC said "these shares are super risky, and may lose all their value in a heartbeat, I need you to hand me 100% of the shares you say you are selling when you sell them." And Robinhood said no. Because that's expensive.
[a] DTCC says put up $100 per share because DTCC is not a lender either! It has lines of credit with various banks. And the banks bear the risk of the loan defaulting. So they're setting their rates and then DTCC draws from the cheapest line.
