While it'd be fun to take out the atheist triumphalism drum, it's worth noting that the thing being measured is religious affiliation not 'theological position' or 'amount of magical thinking done per day', or 'even the vaguest knowledge of how empiricism works'.
Religious affiliation is quite significant, of course, it's obviously notable that substantially more people both can't be bothered to get their ass out of bed on Sunday morning and are willing to admit that they have no formal affiliation(historically, at least in the US, you might not actually attend all that often, or pay that much attention; but denying association was somewhat transgressive). It's also significant for the hopes of various religious groups to exercise political power through organized bloc voting (the 'moral majority', not that it was ever either, sure isn't going to be done any good by the evangelical protestant numbers, nor is the ability of bishops to bluster during election season going to improve with those catholic numbers.
However, it's by no means the case that religious non-affiliation is necessarily anything other than pure disinterest, or vague belief in supernatural entities(probably shaped by a layman-level understanding of whatever your parents nominally believed, with any overtly objectionable parts left on the cutting room floor). There may also be a story about atheism here; but that isn't really the poll result.
In that sense, the results aren't really too surprising: the liberal protestant and 'cafeteria catholic' congregations have been working their way toward being increasingly irrelevant social activities for years to decades now; some nice people and all that; but pretty light on religion, which meant that they drifted into direct competition with any and all other activities you do with other people, without being obviously more entertaining, conveniently scheduled, or otherwise competitive.
The more conservative groups tended to retain the religiosity a bit more intensely; but they really got burned by their flirtation with state power(let's say roughly Reagan through Bush II in round numbers). They did get some of what they wanted, though not enough to prevent disappointment; but they burned a lot of religious legitimacy in the process. Remember that jewish radical who said that his kingdom was not of this world? Well, it'd be hard to argue that the evangelical power-brokers hanging out at the 'National Prayer Breakfast' and trying to get Washington to do something about homos and abortionists do. Even if your beliefs are fairly strong, and largely 'Christian' in outline, it's hard to avoid seeing the liberal wing of Christianity as increasingly wishy-washy and irrelevant, certainly not worth going to church with; and the conservative wing as dangerously unfocused on the kingdom of god in favor of trying to achieve local political gains.