So wood is an insulator with minor conductivity. The tree being the tallest entity in the area, the lightning struck the tree and began dissipating to ground. However, along the way, it found four more conductive paths. three people were standing and so the charge flashed and conducted through them, The fourth was a person who was literally leaning against the tree. He/she took the longest to fall because current continued to flow through the conductive path (bolted fault) whereas the current stopped flowing when the path of conductivity through the air to the three standing was of sufficient resistance (air) to preclude the continued formation of the arc.
Physics.