Yup, they just fucking fell over and died like that scene from the movie Surrogates, if Surrogates starred elk and people actually saw it. Wildlife officials have already ruled out gunshots, anthrax, and pesticide poisoning. Lightning is an outside possibility, but the radius o' death is too large -- Thor would've basically had to get drunk and fry the entire forest.
At this point, the likeliest culprits are either A) an insect-borne illness known as epizootic hemorrhagic disease or B) bear genies. Seriously, the neighborhood apex predators woke up that day to a goddamn free buffet as far as the eye can see.
And so Bear Cinderella's Beary Godmother spontaneously murdered all the animals of the forest, and they all lived happily ever after. (Including the elk, who were much happier as bear shit.)