The physical embodiment of their combined Devil gene, didn't just leave them, but manifested itself into a "human" state. Fearing that it will wreak havoc upon the Earth. ENTER KING OF IRONFIST TORNAMENT 6!!! Thinking that whoever wins the Tournament must be the strongest, they (Kaz, Jin, and Hei) trick the winner into walking into a different room to "To claim their winnings" When the Tournament winner is locked into the room with the Devil gene manifestation.
There wasnt even a tournament in t6 so it'd be thd iron fist tournament 6, not 7 --- http://imageshack.us/a/img210/9229/ttt2kunimitsubikinibyke.jpg http://www.gamefaqs.com/boards/652997-tekken-tag-tournament-2/63435382