In a matter of days, DeepSeek proved to be so popular as to climb to the top of Apple's App Store, surpassing OpenAI's ChatGPT as the most downloaded app. But for owners of Nvidia stock, and stock traders in general, DeepSeek is on track of becoming a black swan — an unexpected event with extreme consequences.

Wall Street is tumbling Monday on fears that the market’s winners who have feasted on the artificial-intelligence frenzy are under threat from a competitor in China that can do similar things for much cheaper.