That seems like the vague non-committal answer for those who don’t have a specific reason.
Probably the most plausible quote from the Pew article is this:
"The second-most-common reason is opposition to the positions taken by churches on social and political issues, cited by 49% of respondents (the survey asked about each of the six options separately)."
This became evident with the huge difference between denominational opposition on same-sex marriage and the young men and women growing up and going to school with gay friends. They couldn’t imagine opposing something that they thought as harmless.