Some stuttering can be caused by trauma. There are cases of discharged soldiers developing stutters. Or abused children. Most stutterign is genetic though. There are even studies that suggest certain groups are more prone to stuttering. And chances are if you stutter some relative also stutters. Even if it's mild. That's true in my case. Anxiety acts as an intensifier, meaning that it can make stuttering worse. Chances are even if you're alone or completely at ease you'll still stutter, just less so because you can probably focus on speaking and not on the fear of how you'll sound.