I watched the new trailer last night and it seemed like Bruce Wayne becomes Batman because of a Superman incident. I'm not sure I like that one bit. Must re-check trailer.
Edit: unless his employees are his family. Still not sure.
This Batman has been Batman for quite some time, but until the events of Man of Steel is in retirement.
The Suicide Movie has (we presume) flashback scenes of Batman vs The Joker (who is most likely the big bad of SS movie).
Then the suit with 'jokes on you' written on is most likely the reason Batman had retired - IE the Joker killing that version of Robin (Jason Todd).
So the Batman we will see in Batman Vs Superman is basically one who's been there, done that for 20 years and eventually just cracked. However, it seems like Supermans fight with Zod and subsequent events brings him out of retirement in a really cheesed off mood.
The way the trailer is cut is probably somewhat misleading though. IE when batman grapple hooks away from the laser blast - I don't believe that is from Superman. (Doomsday?

- something possibly escaped from Lexcorp in the trailer)
Also, I don't read the comics but I heard that Superman has just been heavily nerfed in the recent ones?
It makes sense to help even the playing field a little. Of course, if Superman wanted to win he could just throw the moon at the earth or something. lol.
may I recommend: