We can only imagine that he somehow had a human moment, and messed up.

The normal move instead of 16...c4 is 16...Nd7 17.Ra3 and only then 17...c4.

The way it was played, it was almost the forced loss of a pawn. He had the option of Qb6, but it left him with a miserable position anyway.


     [FEN "r2qrbk1/1b3pp1/p2p1n1p/1ppP4/Pn2P3/5N1P/1P1N1PP1/RBBQR1K1 b - - 0 16"]
16... c4 $2 (16... Nd7 {Preparing f5 immediately so that the Nb4 can come back to d5 after any trade of pawns.} 17. Ra3 c4 18. axb5 axb5 19. Nd4 Qb6 {And black is doing well.}) 17. Nd4 (17. Nf1 {The computer thinks this is even better since the Nb2 is susceptible to Bd2 forcing Nd3 which will lead to the loss of a pawn.}) 17... Qb6 18. N2f3 bxa4 (18... g6 19. Be3 {And this is very uncomfortable for black due to the weakness on b5, and the unfortunate queen.})
19. Rxa4 a5 {And black's queenside is wrecked.}