Arcane Containers can only be opened by someone on the proper step of the Karazhan attunement quest chain. Originally, the key fragments were in the container. However, stealth classes found that they could stealth all the way to the container and take the fragment without needing to fight anything. In order to combat this, once the container is opened a Fragment Guardian (level 70 elite) will spawn. Once the guardian is defeated, the key fragment can be found on its corpse.