A main cause is wearing high heels or shoes that don't fit properly. Too-tight shoes can crowd your toes, forcing them into a bent position. After a while, the muscles tighten and shorten. Pretty soon you can't straighten your toe.
You may be more likely to get these problems because of your genes. For instance, your risk goes up if you have a certain foot shape, such as a long second toe. Some diseases, such as arthritis and diabetes, can also lead to trouble.