Williams was playing within two weeks of the trade, and had to gain medical clearance for the trade:
http://articles.latimes.com/2009/mar/06/sports/sp-elliott-kings-stars6
"He was greeted at Staples Center by team doctors and an X-ray machine, and he's scheduled to see a hand specialist today."
MacCauley and Bure were UFA signings, so totally different, Fraser wasn't actually on IR when the trade happened, and wasn't found to be hurt until after the deal, which is what caused the problem. Cloutier wasn't on the IR at the time of the trade either and played in the Kings first game of the season following the trade. His hip didn't start causing him issues again until a few games into that season.