Nails can become thickened because of trauma. The damaged part may grow out, but if the nail bed has been affected, then the change will be permanent. It could be caused by a sudden blow, such as dropping a heavy object, or stubbing the toe. Alternatively, it could be down to a more gradual increase in pressure, such as that from shoes.
Walking downhill can mean that the end of the toe impacts on the end of the shoe, or playing tennis or squash where you are chopping and changing direction, can also produce pressure on the nail bed. It might be that your 2nd toe is longer than your first, or that your toes claw over, all of which exerts a pressure on the nail bed.
With specialist equipment, nails can be re shaped and smoothed. Not only will they look more presentable, reducing the thickness will prevent pressure sores forming under the nail.