She's done this since we adopted her. Occasionally she'll sit with her butt and tail tucked under and her weight resting primarily on the meaty parts around her tail, upper thighs and lower back. As a result, her hind legs are elevated. I find it adorable. Here's a picture:
Tana is a puppymill rescue and my "theory" is that maybe she learned to sit this way to be more comfortable in the wire-bottomed cage she was living in. Has anybody else seen this in a mill rescue dog? Or ANY dog for that matter??
She does it less and less frequently now- in fact, the picture is from 3-4 months ago and is the last time I remember seeing her sit that way for any length of time. Perhaps now since she is surrounded by mostly soft things, she realizes she can be comfortable just about anywhere (and boy, she can curl up and cat-nap almost ANYWHERE, in the oddest positions!)
Initially I wondered if it could be a sign of knee problems or some other physical discomfort- apparently a couple dogs in her "pedigree" have had luxating patellas….and God only knows what else there could be. But she runs, jumps, sprints and twists better than Jibini or Chloe, has incredible stamina, and never shows any sign of discomfort. I'm pretty sure this is just a cute behavior quirk, and was curious if it was shared by any other Basenjis