How did Big Head get injured prior to the Kun beating?

First we see the driver purposely stopping the truck to set Big Head up, with the latter realizing it after the driver goes into the meadow. The next thing is that Kun and his guys arrive ready to get the baton only to find Big Head missing, and Kun picking up the trail immediately and onward to finding Big Head crawling at the meadow all bloody on his head and left leg. How did that happen?