How can one bad trade send stocks falling off a cliff?

[SteamWake]

In one of the most dizzying half-hours in stock market history, the Dow plunged nearly 1,000 points before paring those losses in what possibly could have been a trader error

Wasent there another issue like this linked to trading software that caused a rather large burp?

http://www.cnbc.com/id/36999483

I just dont understand how one bad commodity trade can send the entire market into a tailspin.

Thank got it still has some resiancy and bounded back. Bet someone somewhere made a few bucks.