When Marvel hosted a media and fan event at The El Capitan Theatre in October to reveal their Phase 3 film plans, most of the focus was spent on next year's Captain America: Civil War. Chris Evans' Steve Rogers/Captain America will be squaring off against Robert Downey Jr.'s Tony Stark/Iron Man, and stuck in the middle of the two sides is long-awaited new character Black Panther (Chadwick Boseman) who is later getting his own movie.
With his home country of Wakanda teased in Avengers: Age of Ultron and his costume revealed at that event, Black Panther is positioned as a key character for what's planned in the Marvel Cinematic Universe, but how he fits into the plot of Civil War remains somewhat of a mystery.
The official Captain America: Civil War synopsis, released as production began on it last week, teases "another international incident involving the Avengers" that "results in collateral damage." The vague description could lend itself to an event that involves Wakanda but according to HH, there might be another way T'Challa a.k.a. Black Panther gets involved. They claim that T'Challa is actively hunting down Bucky Barnes a.k.a. The Winter Soldier.
Since Cap (with the assistance of Sam Wilson a.k.a. Falcon (Anthony Mackie)) is also searching for Bucky, we can start to see how there will be conflict between the new Avengers team and Black Panther. We don't know the why of it, but if Black Panther is on the hunt, that means The Winter Soldier did something to Wakanda or T'Challa's family. Some fan theories speculate that The Winter Soldier, when assassinating targets for Hydra, could have taken out someone close to Wakanda's royal family, perhaps even T'Challa's father, T'Chaka, but with rumors that Marvel is looking to cast the old ruler of Wakanda, we can't say for certain.
If the hunt is personal in some such way however, we can start to see how Black Panther can side with Iron Man's faction for the Civil War, at least for part of the film, given the revelation in Captain America: The Winter Soldier that it may have been Bucky tasked by Hydra to kill Tony Stark's parents (and make it appear as a car accident). This plot point remains untouched upon but will very likely be a source of conflict between Steve and Tony.
It's also possible that while in hiding, The Winter Soldier's could be searching for repairs or upgrades for his bionic arm. Perhaps he's looking for vibranium for an upgrade (a la Ultron), or maybe even seeking out the assistance of Ulysses "Klaw" Klaue (Andy Serkis) for materials (again, like Ultron) and that puts him on Black Panther's radar. There are lots of possibilities but they all lead to the same thing: T'Challa stuck between Captain America and Iron Man as governments around the world attempt to hold the Avengers (and all vigilantes) responsible and accountable for collateral damage resulting from their superhero antics.
The inclusion of Black Panther as a central character in Civil War came before Marvel had access to Spider-Man even though at the time of the announcements at the Marvel fan event in October they were in talks with Sony. Marvel Studios boss Kevin Feige claims that Spider-Man was still a part of the plan all along, and reports backed up leaked Sony info that Marvel had wanted to introduce Spidey in Civil War. However, by all accounts, it seems that Black Panther is taking over Spidey's role from the comics as the central figure stuck between the two sides.
Both Black Panther and Spider-Man however, get their Marvel Cinematic Universe introductions in Captain America: Civil War before getting their own movies in 2018 and 2017, respectively. Here's a little set video teaser that appears to be Black Panther fight rehearsal (note the stance after the attack):