The "resentful outsiders"

In Marger's text E. Digby Baltzell was quoted saying that the Anglo-Saxon-Protestan minority was feeling pushed out and were "resentful outsiders". In what way are these people now outsiders? Could it just be that after holding the majority of power, and being the clearly dominant group they are feeling threatened by the increased racial diversity of American culture. Are WASP's being made outsiders, or are they simply not adjusting well to having to share power?