My working theory for the growth in the religiously unaffiliated is that they demographic of people who would have been nominal religious attendees a few decades ago. The number of people who attend church weekly (i.e. those with strong religious affiliation) has been basically static for a while now [0]. You could say that there is a difference between those with "weak" religious affiliation and those with "weekly" religious affiliation ;)
I think this migration to no religious affiliation is probably due (or accelerated) by the internet. Sociologists of Religion such as Peter Berger suggest that a group of people holding a particular worldview create a social structure for the plausibility of the beliefs that underly the worldview. Exposure to more groups means you have more choice, and in fact must chose, between worldviews. The internet opens up a much larger number of plausibility structures which will be the most enticing to those with loose religious affiliation.