Why has The Catcher in the Rye been banned in so many schools?

I never understood how this novel has been banned in a lot of places but these same schools allow novels such as The Lord of the Flies. Why is this book so controversial? I know Holden uses some crude language and is somewhat perverse during some parts but aside from that, what's the big deal? I don't get why it's one of the most banned books in the US.