After <14...b4>, black faced this position:

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One key observation here is that black's Bc6 has very limited mobility. If this bishop moves to b5 or a4, it would be defended by the queen, but unfortunately both of those squares double-attacked by white. The only square it can move to is d5, where it is thrice guarded by the Pe6, the Nb6, and the queen.

Another key observation is that white threatens this bishop with b5!

Black evidently failed to fully appreciate the vulnerability of his bishop when he played <14...Nf5?>

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This Nf5 is attacked by white's Bd3, and it is defended only by Pe6 -- the *same* Pe6 that's guarding the bishop's only flight square. White alertly took advantage of this overworked pawn with <15.d5!>.

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This brazen d-pawn makes a direct attack on black's Bc6. Black can capture this pawn four different ways, and yet black is surprisingly helpless against it.

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If <15...Bxd5> the bishop looks safe. It's guarded 3 times (by Pe6, Nb6 and Qd7) and attacked just once (by Nc3).

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But then comes <16.Bxf5>, taking a knight and now the Bd5 is attacked twice (a discovered attack from the queen). If black recaptures <16...dxf5>, the bishop's Pe6 defender is gone. It still seems barely okay since it's attacked twice and defended twice.

However, next comes <17.Bxb6!> removing yet another defender of Bd5, and this time it must finally fall.

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If <15...exd5>

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black now has a pawn on his bishop's only escape square, so white simply plays <16.b5!>, attacking the immobile bishop with a pawn that's twice defended. Again the bishop falls.

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If <15...Nxd5>

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Once again black has a piece on the bishop's only escape square, and the bishop drops after <16.b5!>

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Of course, taking the pawn with the queen is no good, and so black took the quick way out: <16...resign>