One simple reason actually, Margot. She started off quite nice, just your average girl wanting to be popular, wants to have a boyfriend and all that. But who never got all that. But as soon as the whole school (ok almost all of them) went to zombie berzerk mode, and she and her friend were the only ones alive (for now), she started to become cocky. Taking on everything that would make one popular, snorting when her best friend wants a non-cool thing.
Though the rest of the book was quite good. I liked how the zombies stayed within their cliques, and when they bit someone that person became one of the clique (including clothes and make-up). Really nice zombie idea.