I have three hens now each raising chicks (separately). It's interesting to notice the differences in temperaments. Our tamest hen is very laid back with us and only puffs up and does her tough gal display if she gets near one of the other hens (behind a fence).
The smallest hen is the most pugnacious. She fluffs up and shrieks a bit towards me when I work around her and her chicks, but she'll fly in attack at anything looking strange to her...like the jar I use to refill the feeder in her pen. Or the saucer under the waterer when I lift it up. Odd stuff.
The third hen I thought was pretty laid back until we had some friends over to see the chicks. She went into full battle mode, shrieking and puffing out, herding her chicks into a tight circle. She didn't calm down until everyone left the back yard.
I think the hen that attacked the chick was, as others have suggested, simply being protective of her own chicks.
Now that I'm watching our hens raise chicks, I have a lot of respect for their mothering abilities. Three weeks sitting on eggs with hardly a break even to grab a mouthful of feed, a quick drink and a fast poo. Then when the chicks hatch, they spend the whole day looking for food which they then feed to their chicks. When we offer mealworms, a favorite treat of our hens in their pre-motherhood days, I have yet to see one of the hens eat them herself. She always gives the worm to her chicks. We weighed our hens, and each lost about 20 percent of their body weight while incubating, so I know they must be hungry, yet their instinct to feed their chicks is stronger.