I have a theory. It's psuedoscience in that some of it is backed up by science, and some of it is conjecture. I used to stutter, but I don't anymore. My stuttering was caused by orofacial disorders that I didn't know I had, but I always knew something didn't feel right about me. Our development of motor skills is a huge part in how we humans acquire new skills. This is where the conjecture comes in. When we begin to acquire our native tongue, most of us didn't stutter badly, but over time it progressive got worse. The motor memory on how to make the sounds in our native languages stuck, but as our stuttering progresses, our ability to learn a new language is severely hampered because it is no longer so easy to say new sounds. The stutterer is still focusing on how to make sounds and not stutter than just thinking and feeling the new language.