Rescued Pekin Duck dies...questions

A friend of mine has worked for a month to get this lady to release two pekin ducks to my friend. Finally, last Friday the lady agreed and my friend took possession of them. I picked them up from my friend on Saturday as she did not have room for them. I kept the pair separate from my muscovies and chickens both so they would have time to settle in and as a (not complete) quarantine.

Today, the female died.

I didn't see that coming. They have been eating and drinking and enjoying the kiddie pool (they didn't have a pool nor were they getting enough food at their previous home) and seemed happy and there was no sign of sickness. But, of course, we know that animals tend to hide their sickness.

Now, I have to wait and see if the male survives.

Thoughts on what could have killed her? Should I have been more careful with how much food they ate? or what type of food they ate? (I was feeding poultry grower, 20% protein, crumbles. Yesterday they also got some kale and mustard greens.

Thoughts on possible disease spread to my greater flock?

Well, this may sound too ghoulish, but I'd pluck her and cut her open. I'd pay special attention to the neck and work my way on down the Digestive track. Pekins can be particularly gluttonous. A slug or a bunch of dry feed could have been aspirated. I imagine that the crop could become to full and cause complications. Anyway that's where I would start if I was really curious.

Once saw a similar problem on a friends goat farm. A new healthy doe suddenly dying for no reason within 48 hours of arriving. Never did find out what killed. Big hole in tough ground that. I would've eat but the owners would have thought that barbaric.

Anyway, just my thoughts, there are defiantly people with more veterinary experience than me on this forum though.