Possibly the driver was traumatized and had to come to grips.  Another possibility given his previous DUI, was he needed to sober up since if he was intoxicated suspect there would be a whole lot more charges than 'failure to render aid'...like vehicular manslaughter.  Alternatively the person walking may have falling into the path of this vehicle which we just don't know given lack of information like what the witness saw.  The investigation continues.