tire cupping causes
Tire cupping is usually caused by worse tear, wear, and pressure on the surface of the tire. Tire cupping is sometimes referred to as tire scalloping. It is an uneven wear pattern that looks like little dips around the tread area. When there is cupping, you might notice a slight rumbling sound as the vehicle moves.
The main cause for tire cupping is worn suspension allowing too much freedom for the wheel to bounce and vibrate, specifically shocks/struts and bushings. If your wheels are going out of balance, this issue is greatly amplified.