As with most types of cancer, the exact causes for ovarian cancer are not well known There are, however, genetic risks (associated with the BRCA1 & BRCA2 mutations).

I am pasting below the corresponding excerpt (under "Etiology") from our Ovarian Cancer Guide at CareAcross (Ovarian Cancer):

The exact cause of ovarian cancer is unknown, although a number of possible risk factors are thought to be involved. Risk factors are related to two major categories: menstrual cycles and family history.

The ovary is subject to a repetitive hormonal stimulus during the reproductive years. Suppression of the ovarian function, as in pregnancy and puerperium, or breastfeeding, is associated with decreased incidence. The use of combined oral contraceptives is thought to be protective, as studies have shown that users have a 30-60% smaller chance.

The more cycles a woman has over her lifetime, the higher her risk of ovarian cancer. Thus starting her period at a younger age (early menarche), ending her period at a late age (late menopause), and not getting pregnant (nulliparity) are all risk factors.

The risk of ovarian cancer increases with age and most cases occur after the menopause. The median age of diagnosis is 63 and older women have worse prognosis. However, approximately 25% of cases are diagnosed between ages 35 and 54.