Though there are theories of why stuttering occurs, there isn't any proven reason. Some attribute it to a stressful event in life at a young age, there are some links to heredity (but no proof), and then there are theories of neurological reasons, e.g. a disconnect/anomaly in white matter tracts (which send neurological signals from one place to another) between your motor-speech areas of the brain and language centers of the brain. So, in short, nobody knows for sure. Most therapy for stuttering, for adults, is focused on acceptance of the disorder and learning techniques to stutter more fluently/smoothly. If it's something you're worried about I suggest you look into some therapy. Especially for a mild stutter there are a lot of things speech-language pathologists can do to help.