There is never any one cause for a significant change. I recommend looking for three. Here in this forum, it appears one of the reasons people chose atheism is because they were taught a literal interpretation of every word in the Bible.
A second common reason is people feel they have learned everything they need to know about the Bible and religion by the time they are twelve. It is interesting that many life decision twelve year olds believe they are headed change as they mature, but the decision they made about faith/religion sticks.
I am up in the air about the third reason. Some feel going to church or bible study doesn't reach the level of entertainment other pursuits offer.
Those who look at parishes and faiths that have the lower drop-off rates say these churches/synagogues/temples are more challenging (or demanding) of their people of faith.