Imagine you’re in a box. Just a box. Chicks hatched outdoors by a mother hen start exploring their surroundings within a couple of days, hearing sounds, digging, scratching up little bugs, playing in dirt, trying out those stubby little wings, and just dashing under Mom for a quick warmup. They aren’t awake with lights on them 24/7. They have things to do, things to learn, a world to conquer - they are stimulated and honing their instincts at an early age. Indoor raised chicks have a box with tall sides and each other. They get bored. They want to use what they are programmed to do but have no way to do it, so they find their own entertainment, and sometimes that means being a bully. Pecking on the other chicks is a reaction to that boredom.
There are things you can do to try to help, but chicks develop long lasting habits at surprisingly young ages. If you can dig up a clump of sod from your yard or garden - weeds, grasses, pebbles, sand, roots, little worms and all - that can help, and as they break it down they’ll love to dust bathe in the loose dirt. Big, shiny marbles in the bottom of their brooder gives them something to peck at that moves....you haven’t laughed until you’ve watched a game of “chick soccer”. Some say a mirror will draw their attention. There are ways to reduce, if not totally stop, unwanted behaviors. Boredom is just an inherent risk with brooder raised chicks, and most of us have seen it first hand, so don’t feel bad!
But your first priority should be making sure that the injured chick isn’t showing blood where she’s been pecked at because that will draw more pecking at it. Treat the injury if she needs it, then look for something to entertain the chicks! Good luck!
And welcome to BYC - happy to meet you!