Well, that's definitely a new one to me. I guess there are a couple of possibilities. One is that they have a very short attention span and have to mouth along with everything you're saying to keep up. Another is that you've just asked them a question, and they're mentally repeating it to themselves because it didn't register the first time. Either way, they're a little slow. I'd suggest getting them a cup of coffee or waiting until they're fully awake to talk to them. If they do it all the time, this person is probably not someone you should be trying to have a conversation with. Either they have some sort of mental deficiency, or they're five years old and mocking you.