Dow Hits 3:50 P.M. Cliff Again

For much of the last hour of trading today, the Dow was up a little, down a little, mostly neutral.

Then, that 3:50 p.m. witching hour arrived again and the Dow fell off the table, closing down more than 200 points, more than a 2 percent drop.

Here's one explanation of why that happens, as it has happened several times in the past weeks.

-- Frank Ahrens