They are blaming mainly an announcement by the Fed.
http://www.reuters.com/article/us-usa-stocks-idUSKBN1771FG
Wall Street falls as Fed minutes reverse earlier rally
Wed Apr 5, 2017 | 5:40pm EDT
Wall Street ended lower on Wednesday after a late-afternoon reversal following signals from the Federal Reserve that it could change its bond investment policy this year, quenching a rally sparked by a strong private-sector jobs report.
Also investor concerns about the Trump administration's ability to deliver promised tax cuts were intensified by comments from lawmakers on deep divisions in Washington.
Most Federal Reserve policymakers think the U.S. central bank should take steps to begin trimming its $4.5 trillion balance sheet this year as long as the economic data holds up, Fed meeting minutes showed.
The minutes also showed "some participants viewed equity prices as quite high relative to standard valuation measures."
Either it scared participants because of talk that sounds like maybe (the stock market) is bubbling, or there is some thought that the normalization of the balance sheet is going to harm growth ... or we are going to get more (rate) hikes than already expected, said Janna Sampson, co-chief investment officer at OakBrook Investments LLC in Lisle, Illinois.
In a heavy volume trading day, The Dow Jones Industrial Average .DJI ended down 41.09 points, or 0.2 percent, at 20,648.15, the S&P 500 .SPX lost 7.21 points, or 0.31 percent, to 2,352.95 and the Nasdaq Composite .IXIC dropped 34.13 points, or 0.58 percent, to 5,864.48.
The Dow posted its largest intra-day downside reversal in 14 months in Wednesday's session after shedding a gain of more than 198 points to end near the session low, which was a drop of nearly 50 points.
. . . Read more at http://www.reuters.com/article/us-usa-stocks-idUSKBN1771FG