Fans boo because they think they're supposed to. With increased attendance, you get an increased number of people who "don't really follow the team but they're doing well so why not go to the game" (yes, please come to the stadium!!) so.. those people have it in their minds that you have to boo former players, or players who don't always perform up to their standards. That being said I love the fact that the more fans are coming to games, and it's their right to boo because they paid for their seats. But the reason your hearing boo's I think is because these fans expect (and they should) a winning team. When one of their "winning components" doesn't produce they feel that they need to boo because that's how sports work and that's what makes players perform. *Edited because I can't proof read.