Could someone please give me an explanation for this pullet's behavior?

'Cause there is something weird goin' on in my coop or something. :/

My 10mo Easter-egger, Cressida, is a pullet for sure. She hen-squats at the slightest sound and I have witnessed her laying an egg. I know her eggs when I see them, and she is still laying as far as I can tell.
Wynne is also an Easter-egger and she is also definitely a pullet. But she isnt the focus of this post, so no more needs to be said here.

Today, while waiting for the last of the hens to come down off the perches, I was standing out in the yard having a chat with the girls when I noticed something peculiar going on between the previously mentioned pullets. Cressy was pecking at Wynne's head. Now, my girls tend to get all feather-picky at this time of the year when they're anxious to start free-ranging, so I wrote it off to that. However, before I could turn my attention elsewhere, Wynne squatted and, of all things, Cressy mounted her! There was even the same tail-end action as is performed by a rooster! Just a second later, Cressy, herself, was squatting, which allowed Wynne to slip out from under her.
So what I'm wondering is, well, what the HECK? Why would it ever occur to a pullet to do something like this? Is it because my flock doesn't have a rooster in it? Is it just a dominance thing? Am I not being open-minded enough about my pullets' choices?

And my apologies if something like this has already been posted. It was just weirding me out.

 

Haha, thanks for the reply! I wasn't too concerned, just wondering why.

I also just noticed that someone else posted about the same thing just under 12 hours ago. How odd is that! 10 month old pullets and everything, too!