Its weirder than you think, most of them don't even realize they're doing it. I've been a physio for years and the amount of women who have come in with random back pain in their late 20s/early 30s would amaze you. I even had to teach my girlfriend to stop doing it. They also do it when sitting down, depending on where. Its why they prefer sitting on beds, because it allows them to slouch more. On a normal chair, spine bent, ass out, by default.
Basically nature itself has decided that constantly sticking your ass out is OP for breeding success chance.