It was stuff left over from the first Gulf War.
From: http://seattlepi.nwsource.com/national/1107ap_iraq_yellowcake_mission.html
Israeli warplanes bombed a reactor project at the site in 1981. Later, U.N. inspectors documented and safeguarded the yellowcake, which had been stored in aging drums and containers since before the 1991 Gulf War. There was no evidence of any yellowcake dating from after 1991, the official said.