Like some people with an actual neurological stutter find themselves fluent when they get really mad or as you said life or death. It’s weird how the brain works. But I stutter when I’m alone, with my parents / family. I personally don’t have anxiety I don’t get anxious when talking to people & yet I still have stutter ever since I could talk my parents tell me. For me & for many others it’s neurological nothing we do about it besides a whole lot of practice. & even then we won’t ever be 100% fluent.