This is normal chicken behavior. It's called "The Pecking Order".
Each chicken in a flock has a rank from "The Boss" to the "Lowest Chicken". Everyone else is ranked somewhere between the two. But this social system is fluid, always subject to changes. When one individual decides to get a little ambitious, she challenges the hen for the rank above her. This is most often so subtle we aren't even aware that any negotiation has even taken place or that today a chicken with a low rank yesterday now has a higher rank today.
Once in a while, a "negotiation" turns aggressive when neither chicken is happy with her rank. Then a fight ensues. The fight can last a second or drag out to a minute with some feather loss and perhaps a bloodied comb and wattles.
Rarely, though, you might see a hen decide she wants to physically dominate another hen, rooster style. That may involve a surge of androgen hormones in the aggressive hen, and then for a while you may see her behave as a rooster, standing on the back of the subjugated hen and maybe even going through mating motions. This may be temporary or an injury to the ovary may have resulted in this change of behavior. In that case, the hen will not lay eggs any longer and will continue to be aggressive.
I just noticed
@21hens-incharge just pointed out the simplest most obvious explanation - the aggressive pullet may be in fact a cockerel. So, now that you have all the reasons behind such behavior, you can select what fits.
