Energy stocks, Alcoa going negative, and particularly disappointing comments from KB Home on its conference call, which dropped the whole home builder market, were early catalysts.
But there were a series of very quick down moves which saw the S&P 500 move from roughly 2,040 to 2,010 in an hour.
That's a big move for an hour, clearly programs going off. explanations include:traders sold a boatload of calls, President Obama's request to the Congress for authorization to use military force against Islamic State, the end of QE is a big topic and what that brings to stock markets.
Read more: thing to watch is the poor performance of financials. They have performed poorly today and were among the first sectors to drop off.