(to be) coming back is the phrasal verb 'to come back' used in the continuous form. To come back - means 'to return'. I am coming back to see you meaning that someone is returning home for the specific purpose of seeing somebody. I am coming back when I graduate from school means someone is returning (to their hometown, maybe) after they graduate. In these examples the present continuous 'I am coming' is used because the person is talking about their plans for the future. :)