Why 13. Bg6 ?
Best Move Puzzle

my guess is that the bishop was previously on e4, unveiling the discovery on the queen by the rook. it can only go to g6 because that is the only square whch doesn't instantaneously make it a target for the queen.
good thinking, anyways.
That's a nice shot, and well done on finding it, but surely ...hxg6, opening a file towards White's king, is better than ...Qxg6 at the end.
I reached this position out of the Scandinavian in a 5 minute blitz game. I was quite pleased to find this position salvaging sequence considering the time constraints and the pressure I was in.