If they are neglected for 15 years they can end up like this.
https://www.independent.co.uk/news/...ears-found-with-3ft-long-hooves-10473164.html
My guess is is a combo of no movement and no nutrition. The hoof perhaps grows weird to try and support what's happening above? The slipper horses are not worked out even fed most likely.
If the horse doesn't move muscles deteriorate and get used to survive. That effects movement and therefore growth.
Aw that is so sad