One official said they identified “a huge, anomalous, unexplained surge in selling, it looks like in Chicago,” about 2:45 p.m. The source remained unknown, but that jolt apparently set off trading based on computer algorithms, which in turn rippled across indexes and spiraled out of control.
Many firms have computers that are programmed to automatically place buy or sell orders based on a variety of things that happen in the markets. Some of the simplest triggers are set off when a stock drops or rises a certain percent in the trading day, or when an index moves a specific amount.
But these orders can have a cascading effect. For example, if enough programs place sell orders when the overall market is down, say, 4 percent in a single day, those orders could push the market down even more — and set off programs that do not kick in until the market is down 5 percent, which in turn can have the effect of pushing stocks down even more.
In the future, undestanding the “Stock Market” will mean studying the AI’s that run it.