I think your on to something with the fox. I keep a game camera on at night that way if anything is out of the norm I can at least rule out a predator. What GQ said is true our ducks/animals know more than we about danger being around. Hope they calm down soon. and check your pen and coop over...
How about foot prints around the pen area? maybe a predator has left a calling card in the form of prints, poop or urine. and they know it and it has scared them. That's the only thing I can think of other than they are sick.