The two man reasons appear to be education and the Internet. Perhaps the Net is the biggie.
Until now, kids could be brought up in a religious home, reinforced at a church and Sunday school, have their education provided in religion-friendly schools, and thus be nurtured in a religious environment.
Enter the Internet with its social media, chat sites, Q&A sites, and exposure to hordes of people whose ideas conflict with what they've been taught, and their eyes begin to open. The smart ones are likely to question what they're now learning and see how it stacks up against what they have already been taught; others will see that the restrictions that have been placed on them by a religious upbringing are dismissed by the majority of people world wide as irrelevant (only 32% of the world population are nominally Christian).
So the teens, among others, find the blinders dropping from their eyes and are able to take a more informed stance. Some will abandon religion altogether, some will return to it with a more tolerant outlook, some will transfer their loyalties to another religion that seems more relevant to them.