I do that, too, when I'm in a shared chair. I think it's because my feet don't reach the floor in chairs that aren't adjusted for my height. Also, I'm autistic, hyper flexible, and have poor proprioception, so sitting on my legs gives me good feedback on where my limbs are.

My office chair is a kneeling chair, and I tend to sit with my feet where the knees should go. (I swap into office slippers, so it's not outside shoes on my chair.)