Under the cover of the Ne5, Black quietly prepares a decisive attack. White has nothing with which to oppose it.
White is suffering, so he might as well have something to suffer for! More practical chances were offered by 31.Bxa4, although after 31...g4 (Black also has a dangerous attack after 31...f3) 32.Bd1 Kh7!, with the threat of 33...f3, Black should win in any case.
31...g4 32.Rxf4 Rxf4 33.Rxf4 Rxf4 34.g3 Nf3+ 35.Kf2 Qxh3 36.gxf4 g3+ 37.Kxf3 g2+ 38.Kf2 Qh2 0-1