Why does beets juice causes kidney stones?

Oxalate Levels: Beet juice has high levels of oxalate. If you are prone to forming oxalate stones the beet juice could hasten the stone formation.

Beets and oxalate: Beets are rich in oxalate. Although many foods contain oxalate, only nine foods are believed to increase importantly in the urine and then promote kidney stone formation. They are: beets, spinach, rhubarb, strawberries, nuts, chocolate, tea, wheat bran, and all dry beans; to limit oxalate stones, limit these foods and increase fluid to 3-4 liters per day -