Traders are warning that many rocky moves could be ahead in the last trading weeks of the year. This, after stock futures took a massive tick down to a five-week low in late trading before recovering, with no apparent reason behind the drop. "The market was relatively thin, and somebody came in with enough volume to move the market," said Rich Ilczyszyn of iiTrader. "Stops were triggered, and it kind of snowballed down from there."

The swift move took the S&P 500 E-mini futures, the , and the Russell 2000 mini futures all to five-week lows at 10:08 p.m. EST Sunday. Then, after shaving off as many as 11.5 points—or 0.65 percent—in a minute, the S&P 500 came back by 4 a.m. EST, and opened higher on Monday morning.

Explanations for the move varied.

"The talk is of a large order, but there is no consensus as to whether it was a 'fat finger' error or just an awful execution—or maybe just price manipulation to see if they could force stops below a technical level," Jim Iuorio of TJM Institutional Services wrote in a e-mail.