From this picture, it appears that a cat’s preference for sleeping on her/his back runs in the family i.e. it is an inherited trait.
This looks like a mother and her kittens. The kittens have either copied their mother or they have their mother’s preference for sleeping directly on their backs.
I have seen an individual cat sleeping like this; I think we all have it, but a entire family is rare which is why I have published the photo here.