My guess would be that his eyes read faster than his mouth does. His eyes are reading ahead, and what is coming out of his mouth is lagging behind, and eventually he forgets what the next word is and has to go back with his eyes to find his place. That's why it's annoying to track with a finger, because it slows down the reading in the brain and makes it seem like a babyish pace.
I do the same thing, and so do my kids.
My mother told of when she was in school and the class would be reading something at the usual mind-numbing pace that students read at, and she would be pages ahead when the teacher, trying to catch her "not paying attention", would call on her -- and she'd have to backtrack in her mind to the last thing someone said, find that spot, and pick up where they left off. This is kind of like that, but with your own mouth reading instead.
