Kevin Nash came back to the WWF in 2002 as part of the nWo faction aka the original Big Three that was composed of Hollywood Hogan, Nash and Scott Hall. The three returned at the No Way Out PPV where they interfered in Stone Cold's match against Chris Jericho. One month later by WrestleMania X8, it became clear that the nWo weren't given any chance to succeed or shine as a faction, as Hall ended up losing to Austin even though he was supposed to win as Austin was a midcarder by then, and Hogan ended up being turned babyface after his match with The Rock and despite his heel role, got cheered over The Rock in Toronto.

Meanwhile, Nash didn't even have a match at WrestleMania X8 as he was reduced to being at ringside for Hall's match against Austin and played his part at outside interference attacking referees until he was ejected from ringside. So anyway, after Hall got fired and Hogan turned face and moved to SmackDown, Nash remained on Raw. He wouldn't have any real impact until 2003 when he came back as a babyface during the peak of the HBK/HHH rivalry and became a jobber to the stars before leaving WWE later that year.

In shoot interviews from what I've heard, Nash's grudge against The Rock is because Nash wasn't given any chance to be allowed to clap back at The Rock after Rock insulted him, which to be honest I think Nash wasn't as bad in the locker room as self-destructive guys like Hall, Hennig, Luger, Bret, Sid, Warrior, and late 90s HBK. Hogan did have some sort of backstage clout on his WWF/E return in 02-03 but was mostly putting over the top guys despite being WWF World Champion before the WWF name change to WWE, so Nash probably should have some backstage clout to at least be allowed to clap back at The Rock in a 1-on-1 encounter/promo.