I think probably because I did a straight theology degree, and to get a 2i, you had to be very critical of religion. My friends who kept their faith never got above a 2ii.
We did lots of Biblical study, and of course that unpicked everything I had been taught to believe - Moses didn't write the Bible, It was the 4 OT types of writers, Genesis 1 and 2 were written by different authors, and have different accounts of creation, almost all mesopotamian myths have some kind of flood story, Paul only wrote 1/3 of the Pauline books. Added in the Hermeneutics lessons which then showed how each little bit of the Bible could be related to a social need in Israel at the time and so on... Oh, and the study of the 'other' gospels (Thomas, Mary etc) which give a quite different picture of Jesus at times. Constant questioning / undermining of the things I had believed to be true.
Then there was the Church Doctrine and History lessons which showed how so many of Christian beliefs actually come from Greek philosophy and are not really Biblical ideas at all, then there were lessons tracing how these ideas have developed over the course of history, and how the church has not always believed that.
Then there were the philosophy lessons, which again did the whole evil thing, how can we justify God saving one man in a miracle when he lets Auschwitz happen and so on...
I also did some modules, on sociology and psychology of religion (we had the Alistair Hardy research centre at my college, so lots on religious experiences and how they are probably not real...)
So basically, it was just three years of constant chipping away at things you once believed to be true, added in to the teenage existential angst that so many of us went through anyway, that many people just could not reconcile the questions they were asked at college and their faith. I wasn't at a Bible college either, so there wasn't the underlying belief in the Bible either. Many of the lecturers were atheists, even some of the reverends I think!
Sorry for the long answer Miss woowoo - but you did ask