Hen mounting other hens?

[Wee Timmy]

Today my 3 year old black sex link hen and my 1 year old light brahma hen were foraging together in the yard when the brahma squatted down in the submissive pose. The sex link proceeded to grab her head feathers and mount her just like a rooster would. They 'mated' and the brahma shook it off like they usually do. The sex link layed an egg later in the day. I have read that hens will sometimes do this to assert their dominance when there is no rooster around but there are two Roos in the flock. Any ideas?

 

[Kiki]

She is just being a "meanie".
She didn't actually "mate" her, just being mean.

 

[Wee Timmy]

She is just being a "meanie".
She didn't actually "mate" her, just being mean.

I have been keeping chickens for years and i have seen roosters mate hens many times, so I can tell you it was the exact same thing. I know it was just a display of dominance but it's strange how a hen would have the instincts to go through the same mating routine of a male.