I have to agree that the sudden change in behavior sounds odd. Are you 100% sure it was her? If something else got them, it wouldn't be unheard of if she was attracted to the already gone ones.....Well, I don't have any now. Long story short, my old lab who is around my birds all the time, decided yesterday that she would eat my whole flock of ducks. She has been around these ducks almost 24/7 since they were hatched 6 weeks ago. She's been around chickens much longer and I've never seen her as much as chase a bird. I don't understand what made her do it. I've got 16 more eggs back in the incubator and a training collar on the way for my old dog. Just makes me sick.
We always get juvenile foxes coming in the spring, that can do some major damage in just minutes.