The explanation I've been told is that when first coming to this country they encountered a lot of prejudice and as they became successful were excluded from the "trappings" of that success by conservative WASP's. Unfortunately, they don't realize that the Evangelical Christians are not the conservatives of old.
Fear of organized groups motivated by other faiths is thus deeply ingrained. Add in contemporary notions of what is fashionable intellectually, and you get an indiscriminate mixture of revulsion and prejudice against Christians.
And let me tell you from personal experience, my "stiff necked" co-religionists are immune to fact and reason in this area.