I still do this, occasionally, as an adult, although my version is less like descriptive video and more like a book being read aloud ("what now?" she asked nervously). I typically did/do it when I'm in the midst of something mindless and the colour play-by-play keeps me somewhat engaged. I couldn't say how often it occurs, because I only tend to notice it as I'm moving off auto-pilot.
It may be that your DS, having noticed it, is having difficulty not noticing it, which is adding a habit element to what is otherwise a natural process. Since it's annoying him, he might want to treat the narration like a habit and focus on consciously breaking the habit when he notices it. Maybe that will help him find a balance that still lets his brain talk to him as needed without bothering him.
