What are the three reasons for the buggers attacking Earth?

"I've heard all kinds of reasons," said Graff. "Becuase they have and overcrowded system and they've got to colonize. Because they can't stand the thought of other intelligent life in the universe. Because they don't think we are intelligent life. Because they have some kind of weird religiom. Because they watched our old video broadcasts and decided we were hopelessly violent. All kinds of reasons."