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.
But only 16.1% of Americans in the study are unaffiliated, and ex-Catholics make up around 10%. There's clearly some overlap, but that's not just it.