When I think of cupping I usually think of the center of the tire being worn out from over inflation. If the inside is wearing or feathering, it can be anything from a warped wheel, the rear may be in spec while at rest, but maybe a worn component at speed is causing your issue. With the previous generations the solid rear beams were bad about bending out of spec and no one wanted to bend them back to correct the camber.

Lack of tire rotation is the biggest cause,  we have seen cars cup the rear tires with a good alignment