ART CASHIN: Traders Don't Know Why Stocks Fell Yesterday, But I'm Pretty Sure The Computers Were Involved

But, traders and investors were taken aback when stocks tumbled on Thursday to kick off the new year.

"The primary stance of trading desks yesterday was one of puzzlement," said UBS's Art Cashin this morning. "Most traders had expected an upbeat start to the New Year, as we've seen each of the last five years. The markets had a different idea."

The Dow fell 135 points at the close of Thursday's trading.

There were various factors that may have contributed to the sell-off. China's manufacturing data disappointed and the U.S. dollar strengthened. According to Cashin, some pointed to the Fed and others pointed to tax strategies.

While no one is certain about what exactly was behind the sell-off, Cashin was comfortable pointing out a revealing pattern.

"One thing seems reasonably evident; the computers were involved in the selling," he said. "The Dow and Nasdaq were both down the same 0.8%. The S&P was down a nearly identical 0.9%. Each index has different components and different weightings. For them to land within an inch of each other suggests rather calibrated selling."

"A further post-mortem would appear to be in order," he said.

In the wake of yesterday's selling, Citi's Steve Englander believes the Federal Reserve will likely make an effort to bring calm back to the markets. Later today, Fed Chairman Ben Bernanke will be among the Fed members speaking at today's AEA/ASSA annual meeting in Philadelphia.

"Most likely outcome - reiteration of dovish tapering stance, buying of risk, bond yields back up with equity market gains, USD advances against JPY, commodity currencies, and probably EUR, but more ambiguous response of risk-correlated EM currencies," says Englander.