That's the abstraction that gets presented to the end-user but it's not how it works under the hood. When trades occur money doesn't change hands immediately, s

That's the abstraction that gets presented to the end-user but it's not how it works under the hood. When trades occur money doesn't change hands immediately, settlement only occurs two days later. The clearing houses require the brokerages to post collateral to ensure that they have the money required to settle the trades. When volatility goes up the clearing houses demand more collateral to deal with the higher risk. If the brokerage doesn't want to post the collateral the clearing houses won't let them trade.