Matt Levine at Bloomberg gives a good explanation: https://www.bloomberg.com/opinion/articles/2023-01-25/nyse-forgot-to-open-yesterday https://www.bloomberg.com

Matt Levine at Bloomberg gives a good explanation: https://www.bloomberg.com/opinion/articles/2023-01-25/nyse-forgot-to-open-yesterday https://www.bloomberg.com/opinion/articles/2023-01-25/nyse-f... Basically at the market open all the requests to buy and sell get matched at the same "open" auction price, then (a second later) the orders get sent to the order book, where the price can go up and down based on size. Because the system didn't think there was an opening, there wasn't the opening auction, the prices went straight to the book and there were large swings in price.