People are fleeing, in fact. An important thing to remember is that in these polls, when they say things like "No religion in particular" they don't actually mean that these people are Atheists. They mean they don't ascribe to a specific dogma, like Catholic, or Baptist. These people often may still think of themselves as "christian" but, for one reason or another, have rejected organized religion.
From 70% to 50% is such a large change in 20 years. For this to be caused by a change in _parenting only_ would mean that probably every child born 38 years ago until today never attended a church. (this data counts adults, so they would only start counting when they turned 18). That seems very unlikely.
