I haven’t heard directly what R’ Bender thinks is the cause of OTD, nor which mehalech he thinks is best, but I can tell you that most of his kids are Lakewood types, definitely “to the right” of CC and Breuer’s.
I’m surprised that you have also fallen into the trap of oversimplifying the issue, although at least you have the seichel to identify the fact that different factors are involved in different communities.
Since you told me one gavra’s opinion, I’ll share with you this yochid’s daas.
First, we can never completely ignore the concept of bechirah. No matter how difficult a situation one has had to deal with, no matter how bad the mistakes the rebbeim and parents have made, I can always find you people who have had this harder, yet came through with emunah and Yiddishkeit intact. So its always at least partially the fault of the one who chose to stop following the Torah. (I therefore take the blame they always seem to place on others with an ocean full of salt, because human beings tend to blame others instead of themselves for any wrongdoings.)
However, certainly other factors are very influential. The negative experiences someone goes through can make it more challenging to stay faithful, so it’s certainly worthwhile to explore which factors are most likely to negatively impact a child.
Here’s where I’ll do the oversimplifying: the single most important challenge faced by kids is emotional trauma. I once heard an amazing stat: 80% of kids who leave Yiddishkeit were sexually abused in one form or another. Of course without a comparison to those who weren’t, we can’t accurately gauge the numbers, but it’s still an eye opening number. I think that it’s not the specific nature of the abuse which causes the problem; any type of abuse is dangerous to spiritual health, including physical and emotional abuse. Some types of abuse are more traumatic than others, but a life of emotional abuse by the parents, which can itself take many forms, can make it very difficult for a child to remain frum.
When excessive chumras are made the scapegoat for OTD, I think the main point is being missed. Often, it’s completely a scapegoat, and sometimes, it’s merely a trigger. It’s not the religion itself – not halacha meikar hadin, and not chumra – which is to blame. In some cases, the way it is implemented might be emotionally abusive. (In those cases, I would assume that the emotional abuse isn’t limited to areas of Yiddishkeit, but its a great defense mechanism for a child who rebelled against religion to lay the blame at the foot of religion.) However, healthy parents, with siyata dishmaya and seichel (b’ikar, the siyata dishmaya is needed to give the parents seichel) can, have, and will continue to be mechanech their children, without compromise.