To kinda tl;dr 015a's comment (which you should read, it's excellent), a liquidity crisis, generally, is when you start running out of cash to settle things that require cash, for various practical and/or regulatory reasons. You might have plenty of other assets on your books, but you're short on cash or cash equivalents.

For a brokerage, the T+2 transaction settlement system requires brokers post collateral while a trade is in the process of being settled. T+2 means 2 business days.

On top of that Robinhood does all kinds of promotional stuff that's a drain on their cash position like giving away stock to new users.