Because of rumors of a suicide bomber:

BAGHDAD, Aug. 31 -- Rumors of a suicide bomber helped set off a stampede Wednesday among tens of thousands of Shiite Muslim pilgrims on a Tigris River bridge, killing at least 700 people as panicked worshipers hurled themselves off the bridge or trampled others underfoot, Iraqi authorities and survivors said.

The stampede was the single deadliest loss of life during the 2 ½-year Iraq war. While the disaster was not directly caused by attacks, tensions had been high because of an insurgent mortar-and-rocket attack upon pilgrims earlier in the day that killed seven. Crowding at checkpoints set up on the bridge to search pilgrims for bombs also directly contributed to the disaster, authorities and witnesses said.

"The people when they were at the bridge, more than one person started yelling, and saying, 'The bridge will fall down, the bridge will explode,' " said Khalid Fadhil, a goldsmith who witnessed the stampede. "So the people started running in panic, pushing each other, trying to run away. Some of the people fell down, and the people stepped on them. The others threw themselves off the bridge, into the river."