Could be any number of reasons. Kale continually produces new leaves at the top while the oldest leaves at the bottom gradually yellow and die. So if the plant looks otherwise healthy just remove the lower yellow leaves. Kale likes moist soil so it could be under watering or maybe nitrogen deficiency (nettle or comfrey tea or mulching with compost should fix that.) If none of that helps send me a photo. I've grown a lot of kale:)