Why is Scout fighting with Walter?

This is because he made her "start off on the wrong foot" with her first grade teacher, Miss Caroline. Scout had explained Walter's "dietary affairs" to Miss Caroline. More specifically Scout had explained why Walter had no lunch and would not accept borrowed money for it. This, of course, is embarrassing to Walter. Miss Caroline chastises Scout for her explanations. Later in the school yard Scout beats Walter up for looking bad in front of her new teacher.