The old man’s changes in demeanor are mostly determined by the density of the crowd around him—the fewer people there are, the more uneasy he appears to become. As the narrator declares later on, the old man “refuses to be alone,” and his shifts in behavior illustrate this clearly. He's figuratively alone in the crowd as he buries himself in it, but if he was truly, literally alone, he’d have no way to distract himself from the terrible secrets presumably haunting him. One aspect of alienation is feeling disconnected from oneself. The crowd allows the old man to avoid facing himself, and it’s possible that the old man does the same for the obsessive and outwardly-focused narrator—that is, he provides the narrator with a convenient distraction.