Melissa,
I am going to think "out loud" here and hope something will be helpful.
Maybe it's a coincidence you've had two ducks affected by prolapse so close together in time. If that's the case, it could be laying problems. Sometimes prolific layers get prolapses, sometimes oviduct problems contribute, from what I have read. Nutrition can play a role. I want to write, here, that in my experience, individual ducks have different nutritional needs. Some have trouble effectively handing calcium, some need extra niacin or thiamine, etc. With your flock, it be you have a few sensitive individuals.
If it's not a coincidence, it could be parasites, perhaps. I do not know of a predator that could inflict that kind of damage but I would not rule it out. That's just really low on the likelihood scale, I think.
If it were me, I'd see if I could get a fecal test for parasites, and start everyone on some extra vitamins with electrolytes and probiotics, and if there's a light on at night, turn it off earlier or have fewer lights on. I would see if I could borrow a game camera and train it on the duck area overnight for clues about something bizarre, just in case.