The Elka Antelope was attracted to Ryan when the rock hit it. The attack was not really damaging to the beast. But it disturbed it's momentum. And that made him a target to its anger.
Ryan dismissively thought that he might have the potential to be a tank. That throw was not even intended to actually hit it! He just wanted to participate somehow, but he end up hitting the bullseye anyway.
Only that he is just clearly too weak to be one. A dumb move now that he had to run for his life once again. And he definitely needs to gather all his energy in avoiding the beast's attacks. It is aiming to kill them at any given opportunity, and he don't want too be the first casualty.
