At 9:50am today, someone sold a lot of Apple shares. Probably some fat-fingered intern at Morgan Stanley or something. That one big sell order pushed AAPL down by maybe 1%.
This drop set off a bunch of algorithmic traders. If a stock’s price is dropping like a rock, a reasonable strategy is to sell it quickly lest you get wiped out. That’s called a stop-loss order.
So some stop-loss orders kicked in, and as each stop-loss further pushed down the share price, it triggered other stop-losses, and AAPL was in freefall for about 30 seconds before NASDAQ’s circuit breaker stepped in at a price of $111.27.
After the pause, AAPL’s price quickly righted itself and went about business as usual. But during that one minute of trading activity, AAPL fell 5% and $767 million worth of trades executed, which is about 15 times the normal activity over the course of a whole day.
Machines are supposed to be more rational than humans. The problem is, they were designed by humans. And because many machines were designed to execute similar trading strategies, they end up exhibiting stupid herding behavior, just like humans.