It all started when Superstar Billy Graham went on Sally Jesse Raphael and claimed he was told to take steroids when he was a wrestler. I think that's the start. In any event everyone was suddenly firing on the WWF. Vince was defending the company, then all these other allegations came out, and the federal trial happened... blah blah blah. A few years later Graham stated that Vince never asked him to take steroids & that he was sorry he ever mentioned the issue to the media. I think there was some other factors at play too like depression or an unwarranted grudge or some crap like that.
Also during the trial WCW was trying to grab up Hogan. Vince fearing that he'd sign up with Turner, stated to the press after the trial that he "was very dissapointed that Hulk Hogan didn't tell the truth." Vince probably wanted to discredit Hogan in the public's eye & sabotage a possible Hulk should he go to the competition. Hulk later said that he never intended to go to WCW, but felt betrayed by Vince & as a result decided to sign with Turner.
It's funny to think about it. If Graham had never said that, the trial might not have happened, Vince might not have taken that cheap shot, Hulk wouldn't have gone to WCW, the nWo might not have happened, and the whole wrestling scene today would be radically different.