What causes recurring ovarian cysts?

Hormones: During each menstrual cycle, the ovary produces a few small cysts known as follicles (normal) . Most times, one of these follicles will enlarge and ovulate an egg (it ruptures), while the others go away. Sometimes this rupture may not occur due to a lack of signaling by the body's hormones, and the cyst continues to develop through subsequent menstrual cycles.

See below.: Ovarian cysts usually develop from normal ovarian follicles. After a follicle expell the egg, it turns into a corpus luteum which appears like a cyst on ultrasound. If the follicle fail to expell the egg, it may continue to grow and becomes a larger cyst (more than 2 cm). This can happen every menstrual cycle so that's why they recur.