One way kidney stones can form is when urine produced in the kidneys is overly concentrated with calcium or oxalate  Reducing dietary calcium has been thought, ttherefore, to decrease the likelihood that calcium will concentrate and form additional stones
Oddly enough, for many people the chances of recurrence are decreased by increasing calcium intake
Increasing calcium intake increases the amount of calcium eliminated through the intestines, which decreases the amount to be filtered by the kidneys.