yankel> zionism was one of the most potent driving forces of pulling the youth [and older people too] away from yahadut before the war in Europe
> as it was with the sefaradi immigrants to the newborn state
these are two separate issues:
1) in Europe, I don;t think I have all info but my assumption is that majority of anti-religious zionists were from those who were non-religious lehathilah.
some of the religious Jews were attracted more to RZ which did not mean they were becoming non-religious. If you have better facts, please bring them. It might have been something in between – people who were on the border of leaving religious community moved to Zionism, so it looked like they became non-religious due to Zionism, when in reality they were destined to some non-religious derech
2) with Sephardim – I would agree that the sudden switch from a sheltered to a non-religious environment, together with the government pressure, lead them to less-religious behaviors. But the move was necessary due to physical sakanah, and they probably faired better than if they were to go to other Western countries. At the end, a lot of “non observant” Sephardim are masorti and pretty respectful of Chachamim and Torah and are not joining fights against religion.