What are the circumstances in which Boyd gets shot?

Boyd and Billy have tacos later and after they've eaten, a group of men arrive with the obvious intention of recovering the horses. There's a scuffle and one of the men is thrown from his horse in the melee, breaking his back. As Boyd and Billy escape, Billy knows that the man will soon be dead. The boys stop in a stand of cottonwoods. When Billy goes out to check the countryside, he discovers they're being followed. He tells Boyd they're going to have to "make a run for it." As Boyd goes for some of the horses, Billy sees Boyd's shadow billow out in the back and Boyd falls. It's a second before Billy realizes that Boyd has been shot.