Most likely he had the camera set on auto (not continous auto, just one time auto focus), since the center of focus was on the rider, he half pushed the shutter (for it to focus) unaware that the camera was focusing on the trees behind the jump, thus the trees are in focus instead of the jump/rider. You would have done the same thing except you would have pointed the camera at the jump first to get the focus THEN tilt the camera back up to get ready for the rider.

Congrats on making that article you got on the pinkbike homepage, good stuff, I'm more of a video guy but same general knowledge. One of my videos is "Just Ride - XC" if you want to check it out.