Ok so everyone knows that this batman is an older one. a lot like the dark knight returns batman and if you've watched the batman vs superman dawn of Justice comic con trailer you can tell that a little bit of it is based off of dark knight returns. But I don't think they're fighting for the same reason they fought in dark knight returns. In case you haven't watched the trailer watch it before you read the rest of this article unless you don't care about spoilers about the trailer. I think they're fighting because in the trailer you notice that it goes back in time to the metropolis battle between superman and General zod. It shows bruce Wayne there and he sees superman and general zod crash through a building and all the dust and debris flying across the streets and bruce Wayne runs into it. I think commissioner gordon was in that building and while bruce Wayne was in gotham he saw that they're was chaos in metropolis and went there to save commissioner Gordon and Barbara Gordon. I think commissioner Gordon was in that building and he brought Barbara Gordon along with him because it was probably a business meeting and no one could watch Barbara and then superman and zod flew threw the building and commissioner Gordon died in the building but his daughter Barbara survived and bruce found her. I think that little girl bruce Wayne is hugging after the dust leaves is Barbara Gordon crying and traumatized from the recent experience. Zack Snyder or someone that has to do with the movie said that it is based a couple of years after man of steel and that could prove that Jena Malone is playing an older Barbara Gordon that is now older because it's been a couple years since general zod attacked. I think bruce Wayne blames superman for commissioner Gordon's death and becomes batman to keep gotham safe and then lex Luthor meets batman somehow and uses his manipulative ways to convince batman that it's supermans fault that commissioner Gordon died and of course bruce Wayne probably already blamed him for commissioner Gordon's death. So batman starts attacking superman. This theory also makes sense because there was a rumor that commissioner Gordon was dead in the dc cinematic universe. In the trailer you also see a Robin suit that says hahaha jokes on you batman on it and is hinting that jason Todd was Robin and joker killed him and now that his parents Thomas and Martha wayne, Jason Todd, and commissioner Gordon is dead bruce Wayne is probably angry that he lost so many people and is now using new methods to dealing with criminals and that is why you see that guy with a bat symbol branded onto him because he was probably a criminal and batman is now more dark than ever and is using those methods and that leads to Clark Kent thinking batmans methods are intense and unnecessary. So batman attacks superman and they fight. That's my theory to why they are fighting.