The data show that this has resulted from Catholics disaffiliating with the church, at least in their heads, and not the growth of other religions or denominations.
It looks like New England Catholics are following the "self-secularization" trend of many northern mainline Protestants. 
It's particularly interesting because Catholics-by-birth have elsewhere exhibited surprising resistance to being labeled nonreligious.
For example, while religious surveys in most of Canada find at least 20% claiming no religion, in Quebec around 90% of the population claims to be Catholic (with additional small percents of Protestants, Hindus, Muslims, etc., making nonreligious a very small group indeed). This despite the fact that Quebec is undeniably the most secular province in Canada; only BC would try to dispute that, and even BCers would admit that interior BC is quite religious. Similar trends have been observed in Britain and in other European countries with substantial Catholic and Protestant populations.
But here we have an example of Catholics-by-birth relinquishing the label as readily as Protestants. I suspect it's related to the extreme polarization of religion in the United States; moderate-to-liberal Catholics-by-birth feel isolated from religious experience in the United States by the extremely conservative nature of public religion and have therefore ceased, in many cases, identifying as religious at all. I'm sure controversies around sexual abuse by the clergy haven't helped matters, either.
While you are correct about Catholics separating from their religious label, it is not as if people are dropping it that crazily. Still 75% of Americans claim Christianity and 85% a religion, but it is questionable as to how many of these people are actually religious. I know many Christians who always claim Christianity, but are not religious at all.
