It could be the result of a fungal infection, or maybe just bad diet. Best thing is to see a chiropodist who will give you some treatment for the current problem, and some advice on how to prevent it.
I've discovered that happens to me too. It has a lot to do with the dampness/driness of the environment your foot is in and for how long. Mine get really dry, so I soak them in moiturising stuff to make sure tehy don't crack again. Also, check out what kind of shoes you wear, some of the artificial fabrics tend to leach moisture ot of your skin... The previous answers are also worth checking up on. I think I will!