[chrisbec]

Doubt that you have a bent frame unless you were in an accident or hit something like a curb drifting sideways, but it is possible. When you lower the car the rear wheels become toe-in and the camber goes more negative, basically the suspension geometry gets screwed up. When you carry a heavy load in the trunk the car sits even lower exacerbating the toe-in and camber problem.
The toe-in is what really wears down the tread on your tires and kills them because they are being drug across the pavement (sideways by a minute amount) for every revolution. That is what kills the tire tread and causes the most tire wear. Negative camber also is not good either and it will cause wear on the inner edges of the tread.
There is a stock replacement trailing arm bushing that can be installed in the trailing arm hinges to help correct these problems. It would probably have to be done by a BMW dealer or a shop that is knowledgeable with these trailing arm suspensions. There are other camber/toe kits out there also. There are poly eccentric bushings (most people complain that they squeak). There is also an eccentric bolt kit that can be installed on the 4 hinges of the axle carrier. In my opinion this is the best solution and gives you the most adjustment for camber and toe, but it is not easy to install. The brackets that are used with the bolts need to be welded to the axle-carrier and the hinge holes need to be slotted. This kit will require some fabrication skills which is something to consider when you decide what to do to fix this problem.

08 SET RUBBER MOUNTING ECCENTRIC D=30MM 2 33329058822 $33.38