Tons of people who stutter have relatives who stutter, so it really seems like there's something genetic about a lot of cases even if no one's been able to pinpoint any specific gene. Also the fact that most people who stutter don't when they're alone and stutter more when feeling pressure or in stressful speaking situations says it probably has at least something to do with anxiety and our perceptions of how much we'll stutter. Seems like there might be physical things involved too, like problems with breathing and timing, since things like singing or speaking in unison aren't usually a big issue. It's honestly kinda weird that the causes of stuttering are still so mysterious and that more research hasn't been done.