Sometimes, fans boo a player that is opposing their team because they know that the player they are booing is good. So basically in the words of a Cardinals announcer "they are booing Pujols because he is good" when they were on an away game.
Another reason fans boo is because the player did something good for his team, but bad for your team. Such as, Hit a homerun, made a diving catch, or ran the catcher over in a rundown to home plate or something along the lines of that.
the reason fans boo their own players is if a player on their team commits a big error and costs them the game or something. Or if they do something bad off the field. Basically they have to do something that costs them the game or gets them in trouble.
there are many reasons why, way too many to list.
Personal Experience from watching baseball.
· 1 decade ago