Quick summary (IIRC, you better go read corresponding articles): The bet that triggered the cascading blow up was Viacom CBS, on it announcing a surprising new issue of stock. The trade was actually structured using opaque CFDs, with Archegos Capital having margin with lots of different banks to achieve higher maximum leverage. As Archegos had been unable to meet the margin call, Goldman started a fire sale ahead of other banks. That caused the other stocks in the position to start tanking as well, and naturally panic broke out with all banks involved trying to liquidate ASAP. In the end, the fills achieved were significantly worse than what the stocks had been trading at market for prior to the event (hence catastrophic losses).