why did the two men have such a difficult time when they tried to harness Buck to the sled?

Francois brings Sol-leks to the position of leader instead. Buck is furious and springs upon him. He will not let Francois harness the team. Francois retaliates by bringing in a heavy club. Bitterly snarling, but remembering his lesson at the hands of the man in the red sweater, Buck stays just out of reach. Both men try to catch him, but Buck will not relent. He wants the leadership.