I think it is the instilled "Stop Loss" idea. Rule #1 is Do Not Lose Money.

A stop loss theoretically gets you out before the loss gets to the point that it is so bad that there is no longer a reason to sell, but to maybe buy or cost average.

Another reason is some folks may already be way in the green zone, and being 10% down on the day brings their overall gain down only a touch. So, why not just get out now from this horrid Market and be safe in cash, then wait for a bottom to buy back in.

That's just the last pair of pennies rattling around in my pocket talkin'.