I think anxiety exacerbates it for most, but stuttering runs in my family. And I'm talking great great grandparents who I never met, not people in my immediate family who I might have "learned" it from like a nervous habit. So I think there can be a genetic/neurological component as well. At least for me. I also think it's possible that simply talking to yourself vs. actual communication with another person (or even speaking on a recording) can involve different areas of the brain. Not that they can really point to anything at the moment.