Besides for the ‘anger at Hashem’ and loss of belief due to the lack of open and speedy divine help, I’ve come to realize another cause for those who turned away from the path of their parents.
During the war, nobody was actively practicing Judaism. They couldn’t. After the war, many people found themselves technically not Orthodox. It was a decision to become again a practicing Jew.
One more point. There were those who became convinced that the world is indeed Hefker. But those who were ‘angry at Hashem’ could have been brought back with the right care. They didn’t lose their belief due to logical questions but rather it was their only way to release anger.