Gutzon Borglum was the sculptor and was involved with the KKK. He was chosen because he was the sculptor of the "Shrine to the Confederacy” which was the inspiration for Mount Rushmore.
Guess it shouldn’t be too surprising given the way the land was taken from the local tribes despite it being sacred.
Credit to this comment by u/alcoholicorn that drove me to look it up.
The whole project was essentially about destroying a culturally significant landmark. The KKK connection isn’t a coincidence, we built Rushmore to break a people’s heart.