My theory?
They imposed chastity on themselves and completely shy away from any form of sexuality, YET they clearly have innate needs (religious people aren't exactly rational individuals so...).
Then they get tempted by whats always around. Little boys. At first it gets hard for them to resist and once they do it, they're hooked because of how easy it is.
Now, i'm not condoning it by any degree. I just think that if the church was more reasonable to allow married priests or to allow them to have relationships, you'd see a radical drawdown in these sorts of abuse.
Hell, even religion's views on sexuality are always controversial. Ya'll know even protestant churches face controversy when the pastor is dating around, as if the man isn't...a man.