Turns out Joss Whedon taking over Justice League wasn't as out of left field as it seemed. The director had already been working with Zack Snyder to write some additional scenes ahead of the film's release in November and will now simply be taking on the full weight of directing those scenes and making final passes on the completed film.
This information was reported as part of the original announcement that Snyder would be stepping away from the film to spend time with his family following the death of his daughter in March. Of course, due to the circumstances of that announcement, the fact that Whedon -- who is also on board to direct Batgirl for Warner Bros. -- was already working on Justice League likely escaped the attention of most fans.
Speaking with Variety, Charles Roven, a producer on the film, said of Whedon, "He was already working with us on some of the scenes for the additional photography that we’re gonna be doing shortly, and it was fortunate that -- Zack convinced him, and he agreed to step in and finish the movie, help Zack finish his vision, and we’re excited about that."
As of now, Snyder is no longer involved in any principle work on Justice League, but all additional scenes shot by Whedon will still need to match the style and tone of the rest of Snyder's work on the film. Roven maintains that despite earlier reports that the reshoots were scheduled in an effort to "save" the massive film -- which brings together all of DC's biggest heroes -- they are instead being used simply to refine it.