I agree withe Ilya, This is a symptom of warped rotors, or inconsistent thickness of rotors. Your rotors must have worn irregularly, somehow. Perhaps the steel was softer in places, and a bit harder in others? It happens. You can resurface them to make them uniform again, or else just replace the rotors with new ones. They're pretty inexpensive in USA, so we usually just replace them, which fixes the problem instantly.