Well, I used to get fluid-filled ovarian cysts a lot. They don't "rupture" as much as just finally drain. The kind I got were caused, essentially, by ovulation -- on a given month, the egg would pop out as usual, but the hole left behind (or pore) in the ovary would not stay open, it would shut on itself, and saline fluid and a little blood would fill in the pore and fill it and fill it, instead of draining naturally out of the hole the egg left behind.