Short timers

We saw a pattern today that has been repeated several times in the last few weeks:  the markets trade up for most of the day, then suddenly crash just before the close.  The down lost a couple hundred points in a matter of minutes; the NASDAQ and the S&P 500 saw like losses.

How to explain this behavior?  Do investors just get sad at night?  Not exactly.  It seems like people's risk horizons have shortened to nearly nothing.  People don't want to hold things overnight, so they dump them near market close.  In the credit markets, we've seen a similar shift towards shorter and shorter credit terms.

Or perhaps there's another explanation I haven't thought of--readers invited to offer alternatives in the comments.

This article available online at:

http://www.theatlantic.com/business/archive/2008/10/short-timers/4277/