Not the skull, but probably the scalp. Our scalp is made up of skin, fat, and muscle. When you press a rigid object against it for hours every day, that soft tissue temporarily compresses. It happens to my kid who wears headphones for gaming. It's the same mechanism that leaves red marks on your nose after wearing glasses, or grooves on your ankles after wearing tight socks. Wash your hair, give up on the headphones, and it'll return to normal.

The QC2 are about half the weight of the AirPods Max, and apparently the mesh in the AirPods Max band sags, and allows the metal bars to "dig in" to your scalp. Enough to cause irritation, but 400 or 500 grams resting on your head can't mess with an adult, developed skull.