The main cause of the Super-Human Registration Act was that Maria Hill decided that all superheroes (active, passive or neutral) must work for S.H.I.E.L.D. because of the Avengers' refusal to work for them.
If activated, it would require every hero who registers to join S.H.I.E.L.D. and become government-controlled heroes. If not, they would be hunted down and arrested.
Hill wanted Stark and his Avengers to be the first to join the Registration Act, which might inspire other heroes to follow, but he refused. Other suggested heroes included the Spider-Man, the X-Men, and the Fantastic Four, whose activities are noticeable to public.
In the aftermath of the Secret Invasion, the Super Human Registration Act is apparently dropped as there no mention of it by the final episodes of the series.
Background in other media
- In the comics, the Super-Human Registration Act is what caused the Civil War between registered heroes and outlaw heroes, which resulted the tragedies of each sides.