Why is it that my eyes keep rolling in the back of my head when I concentrate?

See an ophthalmologist. That really makes little sense. When we close our eyes they roll upward which is a normal phenomena known as Bell's phenomena. However this would not occur while concentrating unless you were closing your eyes. This symptom complex does not fit any pattern so you may wish to get evaluated by your physician.
Need more info! Are you drinking more coffee that has caffeine when you notice that your eyeballs are rolling in the back of your head? This is an important question. We need more information in order to better help you answer this question.