Karolis Springs is part of the Kateen Forest, a sacred forest where Tesla and his apprentice, Karolis, sculpted Owl Sculptures to guide souls to find Goddess Ausrine. I guess the graves have no specific reason other than this place being a graveyard before they started their work, so the souls that might have been roaming there could find those Owl Sculptures to lead their way.
I am not sure if this information is accessible through any book in-game, but I found this when I was translating files from English to Portuguese: Karolis was the best of Tesla’s apprentices, and he desired to be a good Dievdirbys as his Master. One day, he asked one girl that he would do anything if she could give him the power to protect her (not sure how they met, it is not clear in files’ rows). She hesitate, but she ended up giving him such power. Karolis turned into a sacred tree, and he helped clearing out and protect the forest for the souls to find their way. It is said that remnants of Karolis are still found in Letas Stream.
I wrote this from my mind, but the original text is not much different from this.