Based on the ever-leaking set photos from Civil War, I've got a theory on how the Superhuman Registration Act becomes an issue: So far a lot of the leaked set photos seem to have scenes placed in some sort of African environment, which I'm going to assume at this point is Wakanda. I'm guessing the Avengers are visiting Wakanda to discuss the vibranium with T'Challa when Crossbones initiates an attack on the African nation, which sets the events of the film in motion. I'd imagine this happens fairly early in the movie, perhaps directly after the funeral scenes which were shot earlier this week (which were pretty obviously for Peggy Carter). Once the events are set in motion, T'Challa, who doesn't know it was Crossbones who attacked, assumes it was the Avengers, and demands justice from the U.S. government. This could possibly even turn out to be a worldwide thing, and not just U.S.-based.