12The Inciting Incident
In the comic book, a group of young, D-list superheroes called the New Warriors attempt to apprehend an out-of-their-league villain named Nitro while filming a reality TV show in Stamford, Connecticut. Things go sideways when Nitro uses his powers to create an enormously powerful blast, destroying several city blocks
and causing the deaths of some 600 civilians, many of them children. Public sentiment turns on superheroes practically overnight, and the U.S. government is pressured to ensure that superheroes receive training and oversight, so that this kind of foolish irresponsibility can be prevented.
The New Warriors don't exist in the Marvel Cinematic Universe, so the events of Captain America: Civil War are seemingly instigated by the collateral damages that have been resulting from the Avengers' missions. After the casualties and property damage from the Battle of New York and the fight for Sokovia (and likely at least one more incident), it's decided that the Avengers need to be held to higher standards of accountability. Some heroes will agree with this decision of course, and others won't.