When a dog mounts another one's head, he is going to one of the most extreme displays of dominance in the dog world. The head is the highest part of the dog. Bringing the head down brings the dog down from his highest point. It is all about who is higher than whom. It' not sexual as some believe.
Puppies often try to mount each other as a search for where they fit in the scheme of things and sometimes they do it back and forth as it's just a game to them sharing who's on top at a given time. For them, it is simply play.
Adults get into the act as well especially new ones trying to figure out their place in the pack. Here you can see Fiona shaking Kooba off but I don't think Kooba is letting her. But what you didn't see is Fiona giving her the look and then the game ended. Fiona is still figuring out what is fun and her desire to always be the one on top.
This pack works as they do try things as kids often do with each other but they also listen to each other.
Clearly, this is an instinctive dog behavior, a ritual dogs often go through. I recommend to all who are interested to question experts and to study this dog dominance behavior for yourself.
You can correct it if it bothers you.
It's sort of like growling. People react and say - oh they are being 'aggressive' but more times than not it is either a correction from one dog to another - 'leave me alone' thing going on or communication of another sort or a low level fear when it begins. It's all how we handle it or don't that sets the tone for future socialization moments.
Before you react - find out why your dog does what he does and when you do, you may be laughing vs. being frustrated.
My father raised hunting dogs and I can remember him saying to me - think like a dog. Why would one eat what it was supposed to retrieve and the other bring it back. Was one 'bad' and one not or.... figure it out. If you do, you have begun your journey in thinking like a dog. ha!