There are two major changes in the recent American religious landscape, which both start in the 1980s and get to be really dramatic by the 90s but continue into the 00s.
Here they are depicted in graphs created by sociology professor Bradley Wright:
Here we see that the growth of religiously unaffiliated people is quite similar, historically, to the growth of denominationally unaffiliated churches.
Regarding the growth of "generic" Protestants, Wright reports it is "One of the more robust trends in American Christianity," that "the percentage of Protestants who are inter- or non-denominational has skyrocketed."
The growth of the religiously unaffiliated has been much discussed, here and elsewhere (see: Time, USA Today, PBS). Wright, with this second graphs, notes that three different polls, asking different questions, have ended up with very similar results.
Looking at the similarities between the two cultural shifts, I can't help but wonder if they're not best understood as connected. Not that these two distinct developments should be collapsed into one, but that they both result from underlying changes. My intuition is that a good explanation of one development would work, as well, for the other.