Besides a burst/ruptured cyst, what could cause a lot of fluid in an ovary (fluid picked up in a transvaginal scan)?

Ovarian cysts: In normally cycling women (age 12->50 ish), cysts come and go each cycle and are ususally not symptomatic. This is all part of making a monthly egg. The fluid in the ovary is a functional cyst.

Part of cycle: Each month during the menstrual cycle the ovary produces a cyst or follicle that normally ruptures during mid cycle. The fluid from that process is frequently seen during transvaginal ultrasound.

Usually benign but..: While the accumulation of a small amount of free pelvic is related to menstrual cycle/ovluation, there can be other causes of abnormal fluid accumulation. A few of those may include ovarian cancer (particulary if a tumor is see in the ovaries) and bowel pathologies including diverticulitis or small bowel obstruction/enteritis.