Dow closed down 2.2%, May effect?

Yesterday, Dow Jones closed down 2.2% in one day, making yesterday the biggest decline in this year. And this raises fear of double dips depression.

What is the cause? Disappointing economic report? May Sell-and-go-away effect? Or pure randomness?

First, if this is caused by disappointing economic report, why did investors react now? Why didn't investors sell soon after the Japan nuclear disaster? Did previous economic recovery failed to motivate investors. Well, apparently, Mr. Market is very capricious creature. He may overeact or under react all the times. Thus, one possibility for yesterday may be overreaction of investors, magnified by the May effect.

Second explanation is simpler. Yesterday is just another normal day with randomness. It just happened to fall on the tips of the probability bell curve.it is pure happening of chances. No more, no less.