It looks like one of the big reasons is some news from the Middle East. Apparently, Turkey’s parliament has approved sending troops into northern Iraq to go after separatist Kurds (subscription required).
This caused the price of oil and treasuries to spike. Oil spiked on fears of supply disruptions and treasuries as a flight to safety.
And this seems to have taken a good amount of steam out of stocks.
UPDATE (Wed 10/17, 1:30pm PST): Well, stocks stopped on a dime pretty much right after I put up the last post and started heading back up. The Nasdaq reclaimed most of its gains and finished up more than 1% for the day (Nasdaq Chart).