I've been thinking of about this the last couple of days.
We now know that Bruce is returning next summer. We also know that Dan Didio has wanted to kill Dick Grayson since Infinite Crisis, apparently because as Dick Grayson continues to get older, it only ages Bruce more. This is a chance to kill Dick.
Now, I've had no attachment to title stars really before I started reading B&R, because 1) I had no interest with Dick as Nightwing though I realize he is quite likable and loved among many fans, and 2) Damien had been portrayed as a prick. But after these issues, they rock. Morrison's writing has made it work, along with Quitely's art, specifically character designs and use of sound effects. And I realize that Dick's death is going to piss off a lot of people. I understand that.
But here's why I think it happens:
Dick has proven to be a competent Batman, shouldering the responsibility left to him quite well, and is adjusting as the patriarch of this broken family. So after being the man, what else is left for him when Bruce returns? Go back to Nightwing, team ups with ex-Titans. No, no. There's no going back. It would just regress the character.
And also, as of right now, everyone save Tim believe Bruce is dead and gone and there have been no attempts to bring him back. What would be a better impetus than the realization that no one can wear the cowl but Bruce? Dick unfortunately falls in the line of duty, making all of the other members of the Bat family realize Bruce needs to come back.
As for how it happens, eh, Joker takes one look at him and goes, "You're not the real Batman." Then a battle to the death.
This is all nothing but conjecture really, but I wanted to toss it out there.