From the Seattle Times, it is the beginning of the annual hajj pilgrimage for Muslims with 3 million expected this year.

More than 30,000 police and other security forces have fanned out to help smooth traffic around ritual sites that have been plagued with deadly stampedes. More than 360 people were killed during last year's hajj in a stampede at Mina during a ritual symbolizing the stoning of the devil, sparked when some pilgrims in the crowd stumbled over luggage.

360 dead, three and a half times more dead during a religious pilgrimage than the number of U.S. soldiers in a war zone in the deadliest of months of fighting in Iraq.

Problems are so common that Wikipedia has an entire page of Incidents during the Hajj:

On July 2, 1990, a stampede inside a pedestrian tunnel (Al-Ma'aisim tunnel) leading out from Mecca towards Mina and the Plains of Arafat led to the deaths of 1,426 pilgrims.