Some of this could be taken as a bit spoiler-y.
Actor Chadwick Boseman is BLACK PANTHER and he'll be introduced in CAPTAIN AMERICA: CIVIL WAR. Since Civil War is currently filming in Atlanta, Marvel Studios president Kevin Feige must have thought it was a good time to start teasing fans about Black Panther in Civil War, via BirthMoviesDeath.
"It is a role that [Boseman's Black Panther] is still in the beginning phases of taking on, it’s a mantle he is only beginning to take on because his father is older.”
Why Black Panther is being included in Civil War was then the topic of conversation.
“The reason we introduced him in Civil War is because we needed a third party. We needed fresh eyes who wasn’t embedded with the Avengers and who has a very different point of view than either Tony or Steve. We said, ‘We need somebody like Black Panther… why don’t we just use Black Panther?’ That’s how it went in the development process.”
Finally, Black Panther getting involved has its connection to the events in AVENGERS: AGE OF ULTRON.
“Today, pre-Civil War, post-Ultron I think he and his father are saying, ‘A bunch of vibranium just got out of here and wreaked a lot of havoc. Maybe we can’t stay behind these borders anymore, maybe we have to stick our heads out and make at least an attempt to be a part of the rest of the world right now, while at the same time protecting our people.’ That’s sort of where we meet him in Civil War."
CAPTAIN AMERICA: CIVIL WAR releases on May 6, 2016 and BLACK PANTHER releases on July 6, 2018.