There are many theories as to why toenails curl. Besides the most obvious being toenail fungus, others include too tight of shoes, repetitive trauma, vitamin deficiencies, and disease causing secondary effects to the nail like diabetes.
One other reason we tend to overlook is aging. Just as your hair will become kinky, gray, and dry and how your skin with thin and sag, your nails will also become brittle, dry and curl. Take for example, a piece of meat. If you leave it out to dry or try making beef jerky, it will lose its moisture content and begin cracking and curling. It’s an odd example, but it make sense to most people.
Treatments? Home remedies include petroleum jelly (Vaseline),Vicks Vaporub, and Tea Tree oil to help soften the nails. Also, staying away from nail polish with toxic and drying chemicals, routine nail trimming (remembering to trim straight across), and avoiding nail salons all help as well. Surgical treatments include partial or permanent nail avulsions.