It could be that the proportioning valve for the rear brakes has something to do with it or the anti lock system. I know that occasionally when I apply brakes moderately I can feel the back brakes release a little then to maintain the same braking you must apply more foot pressure, effectively making the fronts do most the work. If this is your symptom, I'd say 1) It's normal and 2) Adjust the back brakes, it helps alot. You gotta keep 'em adjusted, I do mine every oil change. Hope this helps.
Tom