The stock market had its worst day in a month after a bond market slide accelerated in the wake of a weak 20-year Treasury note auction.

The Dow Jones Industrial Average fell 817 points, or 1.9%. The S&P 500 fell 1.6%. The Nasdaq Composite dropped 1.4%.

Stocks were trading sideways until 1 p.m. ET, when an auction of 20-year notes saw weak demand. That sent Treasury yields spiking, and stocks falling.