Why do velociraptors eat people? Because it is their nature. Chickens may not resemble their dinosaur ancestors in every way, but they can still be aggressive hunters. Animals that could become prey are often more aggressive to scare away potential predators. And one or two chickens will make an extra effort to protect the flock against threats -- usually one of them a rooster and another the most dominant hen.
It may be her nature, but it also may be the hen if feeling threatened. Give her a safe area for her to nest away from the other animals and make sure you isn't getting scared at night by some roaming predators. Beyond that, just realize that chickens are much bossier and aggressive than many pets, and take that into account when allowing them around other animals. For example, I had to feed my German Shepherd separately because the chickens would take all his food and not allow him to have any.