Babies sleeping with their butt in the air is a natural sleeping position for them because when babies are in the mothers womb they usually are sleeping in the mothers womb in the but up in the air position.
It's just how they slept before and they prefer to sleep that way sometimes once they are born and out in the world.
It's the cutest thing to see when babies are sleeping and have their cute little butt up in the air as well.
I sometimes find myself sleeping with my butt in the air sometimes even when I'm not trying to do it.
When my son was 3 years old he would sleep on the floor sometimes with his butt up in the air as well and it was so cute to see his little 3 year old diapered butt up in the air like that.
He would sometimes sleep like that on his bed as well and he did sleep with his butt up in the air as a baby as well.