It doesn't affect the clearing houses, it affects the brokers.
The clearing houses told the brokers (WeBull, Robinhood) that they need $x of capital to secure the trades, and they simply didn't have that cash.
This is only occurring because of the overall volatility and volume of trades going through particular brokers. Some brokers have been unaffected, presumably because they have more cash on hand.
I don't really understand why they would need to put up a margin in order to execute/clear a fully paid buy order on the exchange. Users promised $x in exchange for stock. RH have their $x. RH needs to make good on the exchange or clearing house. Seems to me that risk runs the other way... Other brokers/clearing house participants might be unable to pay RH users because leverage + a unexpectedly high stock price.
I understand why RH would shut down leverage. After that point, I don't understand why/how stock purchases represent a risk to anyone but the buyer. How is the non-leveraged side creating a risk of nonpayment? I really don't know how any of this works though, so let's leave this all aside. Lets grant that RH/users are at risk of being unable to cover trades.
So what? GME buys on RH create a scenario that risks RH running out of money. So what? Why/how does this risk the clearing houses? It's a $20bn stock. A 2% move in any of the big companies is a $20bn.
It doesn't affect the clearing houses, it affects the brokers.
The clearing houses told the brokers (WeBull, Robinhood) that they need $x of capital to secure the trades, and they simply didn't have that cash.
This is only occurring because of the overall volatility and volume of trades going through particular brokers. Some brokers have been unaffected, presumably because they have more cash on hand.