This study, released today, has significant finding about the decline of Christian affiliation but does not provide explanations for the declines discovered in their surveys. I would suggestion the following reasons:
Personal rather than political reasons
The breakdown of families, which normally have held people in churches
Church-life feeling more hurtful than helpful
Social norms that caused people to say they were religious in the past are fading. People are more comfortable admitting they are unaffiliated.