Salwe vs Rubinstein. Viena 1908. I don´t understand Salwe´s abandonment. 21 plays

A pawn in the loss of material.
Could it be for time ?. I don´t know. I don´t think so.
I look at the pieces on move 21 and I don´t see the logic of surrender.

http://www.chessgames.com/perl/chessgame?gid=1306064

Black has an extra pawn which is central and passed and black has the pair of bishops in an open position. There was no doubt Rubinstein wins that e.g. after Be3. On instead Bxd6 not ...Qxg5 Bxf8 Kxf8 Qb8 checkmate, but ...Bd4 attacking the queen and threatening Qxf2+ and checkmate.