My guess is that, with nonreligious and atheist identification becoming more mainstream, nonreligious people who in the past identified as Catholic (due to heritage, ceremony, etc.) are now identifying as nonreligious. Nonreligious people identifying with a religion has, historically, been pretty much exclusively a Catholic phenomenon, so it would strike Catholicism harder for "no religion" to become mainstream.
Jews.
There aren't enough Jews in the country to change the numbers much. Maybe if you were doing a New York City-area subsample or something it would be worth looking at.
