Reason I'm asking is because while I was fishing yesterday I saw a small button buck breed with a mature female doe. She came out of the woods with him following her and when she stopped he did the deed.
As you can see from this pic he is not fully grown and smaller than the doe.

Last year I saw a mature doe rear up and put a whooping on a button buck that tried to mount her.
Instincts are instincts. Would a 14 year old mount a hot milf?
That said, I haven't seen it often, but it does happen.
Most mature does I have seen would not stand still for that buck. I guess he caught her in the right mood.

like most animals, when the does in season and theres no dominant buck around, the doe will take what she can get.
Here is my take on it.
First of all, the rut hasn't started so she most likely isn't in heat. Secondly, that is probably her buck fawn and she is about to run his ass off. Looks like typical juvenile behavior from what you see young (less than 1 year old) dogs doing. Mounting with no penetration.
But then again, I've seen some strange things, like deer being born in August/September, scraping and rubbing in March.

That usualy only takes place in regions where the adult buck population is extremely low and doe density is high but it does occur.