The rumor mill for Batman vs Superman has been a running rampant in the last few weeks from the first look at Jess Ellenberg as Lex Luthor, to the possible confirmation that Aquaman is only in one or two scenes to a look at Wonder Woman (and the debate on if she fly).. a cameo by the Flash to nearly round out the Justice League team for a follow up film. Other speculation add characters from the Cyborg, to Doomsday and even possibly a cameo of the Green Lantern (not likely) and the suggestion of a return of General Zod.
Many of these rumors, true or false have been at least in part attributed to a series of supposed behind the scenes (read: seen the actual script) articles by Baddass Digest, and now they have let out the biggest rumor to date, the reason behind Superman and Batman’s initial confrontation.
As it is a sequel to Man of Steel, the running assumption was that due to the destruction of Metropolis Batman would believe Superman responsible and go looking for him to avenge the destruction. However now Badass Digest, who claims to have read the script says the Last Son of Krypton gets the jump on the Dark Knight, apparently rendering the Batmobile useless with a single punch at the suggestion of Lex Luther. (who is rumored to bring back either General Zod or create Doomsday using Zod’s DNA.
source: Badass Digest