As people grow up, the lucky ones have sufficient personality left to shape their own beliefs as they wish, and will often abandon religious principles foisted upon them during their upbringing. I've known lots of formerly religious people gain the freedom to make their own decisions when they leave the family home, from all backgrounds, and plenty of non-religious people have also jettisoned many of their parents' morals when they had the freedom to live how they wanted to.
It's quite normal for everyone, not just Muslims, but in my experience Muslims often have the most noticeable turnaround because the lifestyle system they are coming from is so absolute.