They have attacked Shia during Ashoura before, and it appears that this year the Iraqi and US forces have prevented at least one huge attack:
BAGHDAD, Iraq Jan 28, 2007 (AP)— 'U.S.-backed Iraqi troops on Sunday attacked insurgents allegedly plotting to kill pilgrims at a major Shiite Muslim religious festival, and Iraqi officials estimated some 250 militants died in the daylong battle near Najaf. A U.S. helicopter crashed during the fight, killing two American soldiers.'
They want to kill Sistani on Ashoura. Big prize there if they do that.
Update: Zeyad says that this was probably a Shi'i splinter group that opposes Sistani, and that the US was duped by SCIRI into attacking this group of cultists.