Wow that is short! Was the temperature running high? That can cause early hatches. It is also possible they were incubating a couple days before you put them in. Where did you get them? If from your oen birds, is it possible a hen was sitting on them for a day or two and they are actually older? If shipped, warm temperatures during shipping can cause them to start to incubate, although this time of year that shouldn't be an issue. As long as all seem healthy though, I wouldn't worry about it and congrats on the hatch! And we want PICTURES! Please. 
