Because he was the manager and the pitcher of the Peanuts BB team?
So the reference by the Rude One is not really coming from...…..out of left field.
Charlie Brown on his pitcher's mound
Charlie Brown's baseball team
Arguably, the biggest running gag in Peanuts is the neighborhood's dysfunctional baseball team, managed by Charlie Brown.
History
Charlie Brown is the manager of the team and, usually, its pitcher, with other characters of the strip comprising the rest of the team. Charlie Brown is a terrible pitcher, and his efforts often give up tremendous hits which tend to knock him off the mound, leaving him with only his shorts on. However, he shows up for every game with very rare exceptions, and stays even when it starts raining causing everyone else on the team to go home.