These may be many reasons which can cause cracks in the skin. Some of them are:
1. Injury
2. Cold weather
3. Skin infections
4. More exposure to sun
5. Harsh winds etc
Normally cracks in the skin develop due to cold weather. This happens, when the outside temperature is cold and less humid. This causes, the blood vessels to shrink due to which the activity of oil secreting glands (sebaceous glands) in skin is reduced. This makes skin dry and causes small cracks.
To avoid this, you should apply cold cream or moisturiser.