i had a similar problem which i asked the forum about and had assumed it was dominance related - a few advised it could have been excitement and I think they were spot on.
Paddy does it with the new pup and its normally at an excited time as opposed to the odd times he has told her off when he has displayed totally different behaviour. Sounds like its the same kind of excitement my boy has - he almost seems to go into another world when hes there on top - so i hope yours amounts to nothing but exhuberance

It definitely sounds like excitement rather than anything sexual, or "dominance". Dharma does this and she's a spayed female. It's always during exuberant play or when guests come (delightful I know) - it's a release kind of thing.

My wee Paddy no plums had a field day when Asta went into season.He couldn't make his mind up between her and Kizzy(spayed) he went from one to the other,mind you he was a wee bit two stroke and off. It was like a huge doggy orgy as Kizzy wanted to hump both of them too.Asta thought she had died and gone to heaven only to be let down at every turn,xxxxxxxx