didn't explain the issue. And Robinhood did a poor job of explaining to it's customers. Margin requirements are typically 25% - fine in a stable stock, but the volatility in Gamestop would potentially break Robinhood or any security company (if they hold 75% of the risk on the stock.)
https://marketrealist.com/p/robinhood-margin-requirements/
https://www.cnbc.com/2021/01/28/robinhood-interactive-brokers-restrict-trading-in-gamestop-s.html