Stocks fall hard: Dow down 167

New York (May 15)  Fear has returned to Wall Street. Just days after hitting new highs, stocks fell across the board Thursday as the bears came out from hibernation.

The Dow Jones industrial average ended the day down nearly 170 points, though the blue-chip average lost more than 200 points earlier in the day. The S&P 500 and the NASDAQ both fell hard as well, with the S&P 500 close to a percent drop.

Investors rushed into bonds, driving the yield on the 10-year Treasury to 2.48% at one point, the lowest level since Oct. 2013. Bond yields fall when prices rise.

The selling adds to Wednesday's losses, and marks a significant shift from the recent trend. The Dow has hit a string of record closing levels this week, and the S&P 500 crossed the 1900 mark for the first time on Tuesday. It closed today at 1,871.

What's driving the pullback?

There was no specific trigger for the retreat, said Bernard Kavanagh, a portfolio manager at Stifel Nicolaus. He said investors have been on the lookout for a pullback this year and many are primed to sell stocks at the slightest hint of bad news.

"It's more sentiment driven than anything else," he said. "People are feeling skittish after we hit all-time highs just a few days ago."

CNNMoney's Fear & Greed index fell back to a level signaling "extreme fear" in the market.

Some traders pointed to comments by David Tepper, a widely followed hedge fund manager, who said Wednesday that he's "nervous" about the market.

"The tipping point today was David Tepper's comments, which were seen as a siren song. Investors circled the wagon around that," said Mark Luschini, chief investment strategist at Janney.

European markets were also under pressure following mixed economic data from the eurozone. Stocks in Italy, Ireland and Spain fell sharply. Germany and France both ended down 1%.

The jitters were evident on StockTwits, where traders seemed particularly worried about small-cap stocks. The iShares IWM (IWM) exchange traded fund, which tracks a basket of 2,000 small companies, is down more than 10% from its most recent high, which meets the standard definition of a correction.