How can ovarian cysts rupture?

Trauma, over-filling. An ovarian cyst (a liquid-filled lump on the surface of an ovary) that keeps growing can get to several inches in diameter. Such a cyst could rupture from continued enlargement (like over-filling a water balloon) or from blunt trauma, which would likely cause great abdominal pain with internal bleeding, plus an ambulance ride to the emergency room.
See below. Ovarian cyst rupture is a common phenomenon. Menstruating women have rupture of follicular cyst every cycle. The majority of the time this is benign and self limited, with no or mild pain symptoms.