Basically on one of my many many replays of the thieves guild quest line after the final battle with Mercer when the whole place begins to flood Brynjolf couldn't find his way to the surface.
Basically my idea is that the DB has been harbouring all these feelings she deems inappropriate for him. (Maybe their ages are too far apart or because he's a womaniser who would never really love anyone, whatever)
So when the place begins to flood the DB is trying desperatly to get out and once she's out Karliah follows but Brynjolf is nowhere to be seen.
Cue panicking, frantic DB doing the hero thing and going to the rescue hauling his ass out of the water.
Whatever happens is up to A!As, maybe she's angry, maybe he actually starts seeing her as a love interest who he tries to woo etc.

Bonus points if the DB is a Breton or Imperial (size kink)