Firstly, theres a lot of people who are turning against religion and becoming an atheist acriss the board. As for Islam in particular, there most likely isn’t 1 true reason as to why they leave but I believe it could potentially be due to what they stand for. Living in the US I have a different experience and what I learn is much different so bear that in mind and I don’t mean to be racist in any form. But from my experience, most terrorists use religion as a reason for their acts which many of those who follow that religion don’t agree with at all. This forces them to side with religion and terrorism, or to denounce religion. hope that helped!