I know that in areas where volcanic eruptions threaten, there sometimes are areas that get swamped with carbon dioxide gases. The trees die off but not immediately. Sometimes animals and people can just drop dead from asphixiation if they enter such a pocket in a low lying area.
It could be that the animals are figuring this out. But livestock, especially horses and I'd imagine large ungulates like elk are extremely sensitive to ground movement and weaknesses [survival to not break legs I suppose in soft holes]. I've had a horse before that attacked a tree in his pasture for a week before a large earthquake hit. He would back up to it and kick it, strike at it with his front hooves and bite it. We thought he had rabies near the end of that week. But immediately afterwards he resumed his normal behavior. Another mare I had just before an earthquake refused to enter her stall, stood shaking uncontrollably for two days out in the rain and mud outside her shelter with her head hanging low and nose barely touching the ground. We thought she had been poisoned and were ready to write her off. Then after the quake, she instantly became her normal self.
