The traditional explanation of clearing is that it’s to solve the problem where I buy a share of Microsoft from you for $100. The trade won’t complete for a couple days so there’s a risk that either I don’t show up with the money or you don’t show up with the share.

This might happen because I went bankrupt, or MS starts trading for 1000 and you’d rather not give up the share, or whatever. To solve this，there are clearing houses that have collateral requirements to help ensure that the trade is executed as it was supposed to.

Most stocks don’t change that rapidly, so there’s relatively low risk that someone blows up. But GME has both extremely high volatility and is highly overvalued, which makes the risk of someone trying to walk away from it can’t meet their obligations much higher.

Because of the large realised vol in those names (paired with large future expected volatility) the DTCC asked for much larger margin requirements.