From Tekken Zaibatsu Wiki
Heihachi was knocked unconscious when the JACK military unit he was fighting self-destructed. When he came to, The King of Iron Fist Tournament 5 was already over. He went home, without any knowledge of the events that occurred at the tournament, where he was ambushed by the Tekken Force.
Afterward, the Mishima Zaibatsu announced The King of Iron Fist Tournament 6. Heihachi was furious when he heard that Jin had taken control of the Zaibatsu, and he left for the tournament to reclaim control of the Mishima Zaibatsu.