There defiantly needs to be more research into straight leg shepherds. It does look like there are some mutts that have popped up with it as well as a rottie. I do not know definitively that this is a GSD show dog disease, it is unknown what is causing it. We know it is congenital, does this mean it's genetic? So far it's unknown. Hopefully the missing link will be found soon so that we can avoid producing more dogs like Boomer.
I don't agree that sloped backs have nothing to do with this. The abnormality forms in the lumbar region, and sloped conformation GSDs differ from their straight-backed kin from the lumbar region down. Conformation is breeding for appearances and alone does not produce healthy, sound dogs and never will. Talk to any orthopaedic surgeon about our so-called champion GSD, and s/he won't need radiographs to tell you that that dog is a walking vet bill. There are good conformation breeders out there who have their dogs screened for diseases, titled in a variety of sports to prove that they are athletic, sound and intelligent as well as just pretty. They're a working breed, go back to basics, breed them for the jobs that they are meant to do and form will follow. The GSDs that I admire most are from working lines: Sound in mind and body.