long forcing continuation

I don't get it, why can't the bishop just move away? Or even take the pawn?
I mean, I understand that white is crushed and black is going to win the bishop no matter where it moves simply because of white's king position. But I don't understand why a5 is a tactic that wins the bishop. It seems like just one winning move out of all the other winning move.
EDIT: Oh I see, it's because of Rc3+. I didn't see that idea, I just looked at the knight check followups.
Quite proud of this long forcing continuation. Analysis found zero blunders, mistakes or inaccuracies which is very rare for me. Can't seem to attach the pgn as a graphic via my phone.. [Event "jobbistoke vs. regicidal1977"] [Site " Chess.com"] [Date "Oct 6, 2016"] [White "jobbistoke"] [Black "regicidal1977"] [Result "0-1"] [WhiteElo "1692"] [BlackElo "1672"] [TimeControl "1 in 0 day"] [Termination "regicidal1977 won by checkmate"] 1. e4 g6 2. d4 Bg7 3. e5 d6 4. Nf3 dxe5 5. Nxe5 Bxe5 6. dxe5 Qxd1+ 7. Kxd1 Nc6 8. f4 Bg4+ 9. Be2 O-O-O 10. Ke1 Bxe2 11. Kxe2 Nd4+ 12. Kf2 Nxc2 13. Na3 Nxa1 14. b3 Nh6 15. Bb2 Rd2+ 16. Kf3 Nxb3 17. Bc3 Rxa2 18. e6 f6 19. Bb4 Nf5 20. Rb1 Nbd4+ 21. Ke4 Re2+ 22. Kd3 Re3+ 23. Kd2 Rd8 24. Kc1 Nb3+ 25. Kb2 a5 26. Bc3 a4 27. Nc4 Re2+ 28. Ka3 b5 29. Na5 Nxa5 30. Bxa5 Rd3+ 31. Kb4 Nd6 32. Rc1 Re4+ 33. Kc5 Rdd4 34. g3 Rc4+ 35. Rxc4 Rxc4+ 36. Kd5 c6# 0-1