Ok admittedly I've not been at this very long, I've only been Day Trading full time for the the better part of a year and a half, and I've begun to pick up on this curious little occurance with the equity markets. It generally occurs, during the third week of the month following a quarter end. Most often times it occurs on a Thursday sometime around 3pm or so, but it has also occured on a Monday, right from the market open.
Generally it is a big rinse seemingly out of the clear blue sky, violent and very quick without fair warning, generally in the afternoon > 3PM and in response/reaction to a report on CNBC. I highlighted on the $DJI chart the instances of occurrence. CNBC "Talking Head" Triggers have been Carter Worth, Dick Bove, et al. generally it is never the same person.
For whatever reason the market sells off as if on cue and the sell off is widespread and significant. What seems to be consistent is that it appears whenever there has been a significantly positive day/week.
After getting caught by the short hairs last April and seeing it happen in July, October 2009 and January of 2010 I came to the conclusion that this was not a coincidence, but it is either a market quirk, institution situation or rebalancing situation. If I don't take my meds I start to think that "They" are pulling the rug out... whoever "They" is. Since next week is the Third week after the 1st Quarter end I felt it my duty to bring it out to let the sunshine hit it and garner feedback or a plausible explanation. I will publish more analysis documenting this in the coming days, but be forewarned.