It's not really understood that well, but the most in depth explanation I've read has to do with overactive dopamine receptors. That or brain connections between the two hemispheres that are "wired" differently than others. Or a mixture of the two and some other neurological differences. From what I understand the vocal chords being stiff or tightening is more of an effect of that. The brain isn't coordinating the speech mechanism correctly and that's why stuttering happens in the form of repeated sounds or blocking. That's why it's such a hard disorder to treat; things that affect the brain are way too complex for easy answers. It's also possibly why some studies show that dopamine antagonists can improve stuttering to some extent for some people, although not all that well for most and with pretty bad side effects. Like I said before though, there are no definitive answers as to what causes the developmental kind of stuttering right now, only ideas with some evidence.