Hi.
How old are the drakes? They can and will mate the hens particularly if there are not enough female ducks to go around or they are young and their hormones are raging. Whilst there is a risk that the act of mating by a duck will kill a hen or pullet, there is more chance of them introducing an infection into her oviduct that will subsequently kill her. The weight of a duck could also cause injury and the stress of being harassed by them could leave them vulnerable to viruses like Marek's which was my first thought when I read the symptoms. It may also be coccidiosis from that hunched posture. What do the sick bird's droppings look like? She looks like she is on her last legs, poor girl.