I have a Pomchi puppy, about 6 months old, and a golden retriever... our pom does the same thing when she is playing with the big dog. I def do not think it's anything other than playing, maybe even a dominance thing.
I believe this comes from the chihuahua in him! I have three chihuahuas and on a nightly basis, they'll have little wrestling play sessions where they do this to each other. We call it a "butt slam" :) I don't think he's trying to hurt them - it probably is a dominant thing (as it has been with my babes) and it's also a play thing. He just wants to make friends. But maybe also show them who's boss ;)