Casey Burgener, side view, 218kg C&J:
Doesn't look to me like his hips go "back" at all, and it looks like the weight stays over his heels.
If the hips actually move back, something has to counter that movement (shoulders and bar then go forward).
If you were trying to do the dip in a vertical fashion and you felt the weight move towards the balls of your feet, I'd say something is off somewhere. Maybe you were actually leaning slightly back without realizing it, causing the weight to move forward on the feet?