The Superhero Registration Act of Mark Millar's comic is kickstarted by an explosion that tears apart Stamford, Connecticut. Caused by an overeager team of young heroes, the death and destruction causes public outrage and pushes the Superhero Registration Act to the fore. Given the report that General Ross hands to Steve though, the complete destruction of Sokovia at the end of Age Of Ultron will be what forces the US to keep a tighter leash on its heroes. It makes sense to draw on existing continuity instead of trying to cram more into what is surely going to be a very busy films already, and it lends a strong international relations aspect to the story that will likely draw in the Wakandan monarch, Black Panther. Still the US bowing to pressure after the large-scale destruction of sovereign land doesn't seem too realistic, does it? It may be that the impetus feels less real without the US simply protecting its own citizens.