Brokers are required to keep a certain amount of capital on hand to ensure that customers can be made whole in case some sort of risk arises.
The amount of money they are required to keep on hand varies according to the risk of the investment (among other things). The increased volatility translates into higher risk and therefore requires the broker to keep more money on hand.
I haven't seen the exact amount in this case other than hearing total amount of $14 billion and that it would be a percentage of that.
For shorts and trading on margin that all makes sense, but I still don't get why that would prevent retail investors from buying regular shares with their own cash. Where does the broker assume risk in that transaction?
Edit: nrmitchi below explained how the broker is legally required to have some extra collateral on hand in between accepting and clearing the order.
The amount of money they are required to keep on hand varies according to the risk of the investment (among other things). The increased volatility translates into higher risk and therefore requires the broker to keep more money on hand.
I haven't seen the exact amount in this case other than hearing total amount of $14 billion and that it would be a percentage of that.