There are several causes for thick nails(onychauxis).
Repetitive injury to the nail over time or a severe one time injury to the nail.
Fungal infection of the nail
Debridement of a thick nail
Debridement of a Ram' horn nail
The most common cause of this nail problen is a severe blow to the nail. This is followed by an inability to cut the nail. The nail will continue to grow and bigger footwear will be used to accommodate the nail.