rotor warping

Perhaps the brakes are biasing to much to the front (rear brakes not working correctly). Some cheap rotors are prone to warping use premium brake rotors.

When it comes to "warped", glazed, overheated rotors, or rotors with excessive thickness variation it is almost always caused by the rotors being subjected to extreme heat. Around 9% of the time this is caused by binding or sticking brake components such as slide pins, brake pads, or calipers. 1% of the time this is due to a blockage in a brake line not allowing pressure to escape back to the master cylinder. 90% of the time this is caused by the driver resting their left foot on the brake pedal or repeated aggressive braking. Essentially if the rotors have been replaced multiple times and you're still having issues the problem is likely related to your driving habits.