Dogs that sit this way MAY have Hip Dysplasia. I must stress that they MAY have it, but some dogs just like to sit weird. They can usually tell with an x-ray at your vet. The vet told me that a high percentage of dogs have Hip Dysplasia, but there is varying degrees. Sometimes its just mildly uncomfortable, but sometimes it can be very painful for them later in life. Fortunately, theres a few things you can do to help alleviate it. Lower there body weight, keep them from jumping and landing on their hind legs, etc. There's shots that you can administer to promote joint fluid. They are kind of expensive, but apparently they work really really well. Extreme cases of Hip Dysplasia might need surgery.

Source: One of my dogs sits like this but kind of splays one leg out to the side. She was just recently diagnosed with Hip Dysplasia. She's 2 and a half and we caught it early and started the shots. There was an immediate change in how active she is.