When I was 10-11 years old, I had a classmate who would try to imitate my stutter. And he eventually developed it. Then we moved to different schools. Years later, I found him on social media. I asked him about the stutter. He said he joined a drama/theatre group, and now he doesn't stutter anymore. As a child, we are not aware what habits we can pick up subconsciously. Sure in some, it maybe genetic. In others, it could be a learned behaviour that became a strong habit over time.