Niko will sometimes sit on Rosa when I am petting her. I think part of it is that he's not really as aware of his back end as he could be, and part of it is "Pet ME now Mommy!"
Rosa will sometimes stand over Niko. I think it's part of their play, because it leads to a fun game of bitey-face.
If I had to guess, I'd say they are both attention-getting behaviors.
Niko: American Showline GSD 5 years old
Rosa: American Muppet Dog (GSD/Border Collie mix) 5 years old