People do weird things, so someone could be poisoning wildlife. Sometimes people will put out things trying to deal with one kind of unwanted wildlife and end up killing cats, dogs, or other wildlife that accidentally get into whatever was set out. But it could also just be a local outbreak of something that will quickly run its course. Or, occasionally, several juvenile squirrels in an area will die. Without testing the animals it isn't possible to know for sure what is going on.