The idea behind a circle hook is that when the fish swallows the bait it will not get gut hooked. The theroy is that the hook will slide back out the gut and then catch in the hinge of the jaw. Striking the fish hard will not allow this to happen, rather a hook up is established by slowly nudging the drag up to strike and then leaning back into the fish when weight is felt. Like i said, I.M.O this allows kings to much time to find structure. It may be, that you were too eager when hooking the fish, resulting in a poor hook set. Keeping things slow and steady as required with circles is difficult when you have a good fish on. But in saying all of that it may have been that the hooks simply pulled, it happens.