The market tends to act erratically. The last month has been a time of unprecedented economic stupidity over the debt ceiling. This was a self-inflicted wound done by the idiot teabaggers. The market thought that a better solution would come out of this, or more specifically, people who play the market thought that others who play the market would panic, and so they stopped buying long ans started buying short. Trends are amplified by computerized trading systems, that echo the panic of these second-guessers and that caused the drop.
Strange that when the market rises, Obama has nothing to do with it, but when it falls, he is to blame.
Of course, that is not true, as he really has had little influence either way.
When it drops, that is when you buy. When it rises, sell. Buy low, sell high. When there is simply panic, do nothing, assuming you have a balanced portfolio.
But strangely, many people do exactly the opposite.