Ryu had a quote which said 'You must defeat Sheng Long to stand a chance,' which referred to the Rising Uppercut.
Then EGM pulled a prank where they posted screens, designs, etc., of Sheng Long as a character in the same style as Ryu and Ken, except more powerful, and the method to reach him. It was some difficult thing like not touching Bison during the fight with him until the time limit, or something like that, at which point Sheng Long would jump in, kill Bison and you would fight him.
Capcom decided to roll with it once the rumor caught on, introducing Akuma in Super SFII Turbo as a playable character using a code at the select screen and a secret boss who would jump in and kill Bison, provided you don't lose any of your fights up until that point.