Pretty much anything. Any stress not just during brooding but also a difficult hatch or a change of diet/treats. You can think you are raising every batch the same and still have an entire batch every now and then get pasty butt repeatedly while the rest never see it. Whether it will come back or not no one can really say. Sometimes it does, sometimes it doesn't. Just keep cleaning it up and watch the chicks to make sure they are the right temp with plenty of clean water and food.