I think it’s not a mental disorder, by definition. Most people stutter because they anticipate it and fear it might happen. The fear of stuttering is in most cases the actual problem. So I’d say it’s an issue of habit. Paul Stamets describes it as something along these lines: Imagine a snowy hill. You’re riding your sled down and create a path. You ride down this path multiple times until it’s impossible to get out of it. Out whole consciousness is a product of our experiences and environment. Thus, it’s extremely subjective. We just don’t know any other path of speaking, because we’ve gone down the same path -in most cases- for as long as we can think.