A week before the grand spectacle, Jim Ross still had not received his release from WCW. He announced his resignation from the company on February 25, after WCW re-assigned him from commentary to syndication sales (yes, really). As of Sunday night, March 28, 1993 (one week before WrestleMania), Ross was still technically signed with WCW, according to Dave Meltzer's Wrestling Observer Newsletter. There, he would stick it to his soon-to-be former employer.
Ross hosted a Sunday night radio show on Atlanta's WSB station, and on the March 28 broadcast, he announced he was WWE's newest broadcaster. Then he had Vince McMahon, Bobby Heenan, and Shawn Michaels on his program as guests, promoting WrestleMania 9. Since WCW was paying for the show, WrestleMania 9 got some extra publicity on WCW's dime.
The Observer speculates that because WCW removed Ross from its payroll (in the transition from broadcasting to sales, perhaps), it created a loophole that allowed him to sign with WWE when he did.