Of course there is no one reason why people go off the derech. I think the people that say “such and such is the reason” are simply saying that from what they’ve seen themselves. That’s why they say “this is the reason”.
So instead of ridiculing or making blunt remarks to someone who says a specific reason for it, we should understand that they are simply saying that because that is what they’ve seen to be the reason. Of course they know there are other reasons, too.