How does August die?

August dies after Rosie (the elephant) pulls up her stake and slams it into the back of his head. The violent action splits August's head, and he falls to the ground.
After bringing the stake down on August's head, Rosie steps back and gently slides the stake back into the earth. Almost immediately, a herd of zebras trample August's body. The stampede leaves August's body a tangled mass of flesh, viscera, and straw. It is a horrible sight, and when August's wife, Marlena, sees it, she crumbles down to the ground.
Meanwhile, the other circus animals join the herd of zebras. The animals are free because they were released by disgruntled circus workers who were consigned to be red-lighted. In the circus world, red-lighted workers are those who are slated for dismissal from their jobs. In some extreme cases, like in Water For Elephants, red-lighted workers are thrown off the train to their deaths. In chapter 22, this is exactly what happens to Camel and Walter.