Hey mate, there is a researcher here in Perth Western Australia who reckons it's to do with the brain learning incorrect speech when you're young. Basically, it's muscle memory, which is why early intervention is key. She's told me that they are nearly at a point where a cheek swab for a DNA test could show whether a child will have a predisposition to stuttering. Her name is Janet Beilby. She's an awesome lady. Check out her papers on Google scholar.