Most of the Dobes I've owned/fostered have done that. FWIW, I read somewhere the trait was inherited from the Greyhound. Kenya would park her butt in my lap if I sat on the ground. A couple of them liked to back their butts up onto the furniture, to sit right next to me and then turn their heads to look at me with an expression that said, "Well, what are you waiting for? Scritch my butt!"
Yup, Zeusy does this to. He will actually walk backwards and plop his butt on your knee when you are sitting on the couch. Whenever he does this, we just go "beep, beep, beep, backing up Dobe, beep, beep, beep!" He is usually not very amused.