it wasn't just a one way thing, it was a two-way hate fest for a long time at the start of their careers. The first two times they played each other (at a 75K challenger in '95 and then again at a Fed Cup match in '96), Patty defeated Anna in hard-fought bitter contests. After the Fed Cup match, Anna was talking trash about Patty, saying something to the effect of, "if i keep losing to players like that, i'm never going to make it on the tour." When Patty and Anna played each other for the first time on the WTA, at Amelia Island in '98, Anna crushed Patty like 6-1 6-2, and Patty basically went nuts after the match, complaining that Anna's entourage had been harassing her from the stands and some other stuff.
Then they played again at Amelia the following year, which is the well known incident where Patty supposedly tanked. But i have that match on video, and honestly it was more that Anna was just playing great tennis and totally kicking Patty's butt for about the first five games, and THEN Patty starting tanking when she got frustrated that nothing was working for her that day. In the post match interview Anna looked piqued by questions about their personal animosity, and she had the nerve to lie and say that "I've never lost to her [Schnyder]."
They played once more, at RG that same year, with Anna beating Patty by a bagel in the third set, and in the years since then it has been a one-way thing, with Patty making openly bitchy remarks about Anna to anybody who would listen, and Anna basically taking the high road in response, denying that anything ever happened. Patty got quoted in the press saying stuff like, "You should see everybody smiling in the locker room after Anna loses, nobody likes her." and "I have a lot of friends on tour. We all get along except for Anna. But that's ok, who needs her?"
As recently as Zurich '02, Patty said something like, "Kournikova is a disaster of a human being." But in the last couple of years, Patty has changed a lot, she has turned into quite the diva in her own right, and maybe she has finally realized that it's pointless to go on and on criticizing Anna in such a public way - i just hope for Patty's sake that she has finally gotten over this.