Chickens can get more aggressive in smaller set ups, especially after sexual maturity when territory becomes more important.
I'm assuming the whole are they are in is than 3x6 which is pretty tight for that numbers of chickens. Giving them more room or free ranging time may help.
Pullets can get moody around the point of lay.
How big is your set up?
You may need to separate her out within the coop or run if the picking continues.