Dachshunds have long backs because they have been bred to hunt badgers and be able to fit in tight spaces. The hunting background makes them very active dogs, with quite the courageous stance.
As a puppy grows, it also grows out of this sitting stance. Sometime after about 2 years, it will start sitting straight up with the hips under the body.
A small issue like sitting opens up so many other problems with the dachshund. The doxies have an interesting way in which they sit, and an even more peculiar one when they sit up.