this explains it from Wikipedia
On June 14, 2005, the Nationals began a three-game series against the Angels, who were still helmed by Mike Scioscia. This marked Guill?n's first return to Anaheim since being traded. Going into the series, both Guill?n and Scioscia kept a civil tone publicly, each indicating that the past was behind them and claiming that they held no hard feelings toward one another. However, the tensions below the surface were exposed when, during the second game of the series, Angels pitcher Brendan Donnelly was found with illegal substances on his glove.
Donnelly was ejected from the game, and Scioscia then came out of the dugout and exchanged hostile words with Nationals manager Frank Robinson, who had instigated the search of Donnelly's glove. The confrontation led to both teams' benches being cleared as all of the players streamed out on to the field. As he was being restrained by fellow Nationals players, Guill?n shouted angry words at the Angels, a number of whom made it clear that they felt their former teammate had been the one who told Robinson to have Donnelly's glove examined. (Several weeks later, Guill?n would acknowledge that he had indeed done so.)
In the eighth inning of the same game, Guill?n hit a two-run home run to tie the game, and the Nationals went on to win. After the series' final game, Guill?n blasted Scioscia and acknowledged that despite his earlier statements to the contrary, he was in fact still hurt over what had happened at the end of the 2004 season.
I don't got truly no respect for [scioscia] anymore because I'm still hurt from what happened last year . . . Mike Scioscia, to me, is like a piece of garbage . . . He can go to hell . . . I can never get over about what happened last year. It's something I'm never going to forget. Any time I play that team, Mike Scioscia's managing, it's always going to be personal to me.[1]