When this movie was being made, the director Zach Snyder had to step away to deal with family matters. Warner Brothers brought in Joss Whedon to finish directing the movie(reshoots, editing...). I also believe Warner Brothers wanted changes made to the movie as well. The WB last year decided to let Snyder put together his vision for the movie. He did some CGI to give it the appearance he wanted. He changed the musical score. They filmed a few new scenes, but from what I understand those new shots make up less than 15 minutes of the footage. It was mostly cutting room stuff that was added back in.