Why did the Dow just lose 1,000 points?

Here's a comforting answer: No one knows. But everyone is freaked out. Peter McKay offers context:

The Dow went into freefall, tumbling through 10,000, before dropping as much as 998 points, or 9.2%. The biggest closing point drop in the Dow's history occurred on Sept. 29, 2008, at the height of the financial crisis, when the Dow ended the day down 777.68 points, or 6.98 percent. The Dow has since pared its losses but remains sharply lower, down 492 points, or 4.45 percent.

So what's going on? A lot of people are blaming it on a mixture of a computer glitch that made a couple major stocks (notably Proctor & Gamble) look like they'd tanked and fears over Greece. Megan McArdle rounds up some additional theories.

But whatever the ultimate trigger, the drop was too big for the cause to be this uncertain. What you're seeing here is a very, very fragile market. There's so much unknown risk out there -- notably, but not solely, in Europe -- that quick movements are sending everyone running for the door. That is to say, we're seeing the return of financial-crisis psychology, where people fear because they don't know. That's why very calm people like David Cho are saying very scary things.

