Selling in stocks and a surge of buying in bonds—driving interest rates dramatically lower—accelerated Thursday after negative reports on manufacturing and housing, two key legs of the economy.
But the positive momentum in the bond market was already in full swing and has been the trend this week around the globe as traders react to central bank easing, particularly in Europe. Thursday's rout also reverses a schism in markets, where bonds had been moving higher and stocks were also rising, with the Dow and at record highs earlier this week.
Markets were also spooked by comments from widely followed hedge fund manager David Tepper, who was cautious on stocks and said he was not recommending going short but "don't be too fricking long right now." That comment, in various iterations and spellings, sped around Wall Street overnight and greeted many traders in their in boxes Thursday morning.
Read MoreTepper: I think it's nervous time
The 10-year Treasury charted a parallel course to the German bund, which was yielding 1.30 percent. The 10-year yield broke below the psychological 2.50 percent, a level many analysts thought would hold as a floor just days ago. While positive for mortgage rates and other loans, the drop in bond yields has been worrying stock traders who fear it is signaling a weaker economy.