I don't think I've ever paid attention to it until recently when my trainer brought it up. My mare carries her tail high and away from her body when I'm riding, and as far as I've ever seen, does the same thing in the pasture.
She has been fully examined, poked, proded, flexed and tested by a vet and also recently saw a kinesiologist who found her to be in good shape all around except for a little hypersensitivity in her hind end muscles and the muscles in her stifle area. Treated her for this. He found no soreness in her back/pelvis at all. I know that tail carriage like hers can indicate issues there, but this the kinesiologist is very, very good and I know if there was something sore he would have found it.
Between he and the vet, I'm confident that she doesn't have a back soreness issue. My saddle fit her, although I recently bought a new one (that I also know fits), and if by some odd chance it didn't, it would have made her seriously sore by now. So I also don't believe it's saddle fit.
I want to believe that it's just her and how she likes to do things. When I first purchased her, she had a funny thing where her tail bone kind of curled and poked outward and to one side at the tip. That, however, went away after a couple of months. There was no pain, so I doubt she injured it.
I guess I'm just looking for opinions so I can say "this is why", because I'm tired of "WHY is she doing that?" like a high tail is the weirdest thing in the world, and me having nothing to say but "I don't know? Why is it your concern?"
She does a lot of funky things, but by all accounts of the professionals, she is sound and pain free, just quirky and strong willed perhaps.