Ok here's my " Batman vs Superman: Dawn of Justice", reason for Wonder Woman theory. Lets start with few points. Please keep track of Bold points.
1. Based on leaked pics from the Wonder Woman movie we can assume she will have a Captain American-esque type origin. Meaning that her story will start in the past, id say between 40’s and 60’s which would tie in with Linda Carter’s version and comic book history.
2. This being said Wonder Woman is tied to Steve Trevor (military Intelligence) who is her Liaison, love interest and first encounter with Man outside of her Island. So it is fair to say that there is a MILITARY FILE ON WONDER WOMAN.
3. Wonder Woman is tied to Greek Mythology. In Greek Mythology we have Hephaestus who was frequently tasked by Zeus to create weapons and according to some sources the Android Talos. So we can assume that there could be an Artifact used in these tasks.
4. In comic books, Lex has been contracted by the military and Lois’s dad to create weapons to stop Superman in case he ever went rogue. In every one of these cases Lex has misused the weapon and tried to kill Superman. So let’s say that his association with the Senator in the Trailer means he has a CONTRACT TO CREATE SUCH A WEAPON.
5. With this contract Lex would have access to Military tech and FILES, as well as ZOD’S HEAD.
6. As we all now, history has told us that Hitler's obsession with power lead him to fund expeditions around the world to locate and retrieve artifacts of great power tied to ancient mythology. The first use of this in comic Movie lore was Hellboy.
This all being said here is my theory. Like Civil War, Superman is being held accountable for the events of the first movie. This leads to Lex manipulating and cheating his way into a Government Contract in order to create a weapon to stop Superman. With said contract Lex gains access to not only Zod’s head but Steve’s files on Wonder Woman as well. Lex then uses this knowledge to fund a Hitler style expedition to steal/retrieve a certain artifact of Greek Mythology from Wonder Woman's home Island. This theft leads Wonder Woman to assume one of her old alias's/covers on behalf of her Queen, who has tasked her with the retrieval of said artifact before it can wreak havoc on the human world and break any treaties /arrangements forged between her world and ours in the past with the help of Steve. With the stolen Artifact Lex is able to to reach his goal and quench his obsession with ending Superman. Fear of getting caught before the grand reveal, Lex uses Batman in order to distract Superman and the world from his plans. Meanwhile Wonder Woman whose hunt for the artifact has brought her to Metropolis, eventually tracks it to Lexcorp, but before she can take it out of play, Lex manages to use it along with Zod's Head to create Doomsday, and because of this Wonder Woman now feels obligated to help, there for revealing herself to Batman and Superman.
Ok While writing this I happen to think up of another shorter tie in for Wonder Woman Appearance that also makes sense :/
She is tied to Steve, and we have no idea what direction her movies will go in. So we can assume she has relationship with our Military. We have seen her play the role of an Ambassador and that of a Military Intelligence Agent in the comic books. So it’s fair to say she was called in by the military, re assuming her old alias/cover to asses and keep an eye on Superman until a proper decision regarding him can be made, during her stalking of Superman with strict orders to keep her distance, Doomsday shows up and she decides to break protocol and reveal herself to help stop Doomsday.
So what do you think?