Both stock and bond markets had already been having a rough week, and then on Wednesday, Federal Reserve Chair Janet Yellen added to the jitters.
She warned that stock valuations are "generally quite high," and that "there are potential dangers there."
So if you happen to be an investor who wants to buy low and sell high (and really, who doesn't?), then you might take Yellen's comment as a suggestion that it's time to sell.
And that's just what happened: Measures of U.S. stock prices all slipped — down about 0.7 percent by midday.