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/1p1P4/Pnp1P3/5N1P/1P1N1PP1/RBBQR1K1 w - - 0 17"]
17... c4 $2 (17... Nd7 {Preparing f5 immediately so that the Nb4 can come back to d5 after any trade of pawns.} 18. Ra3 c4 19. axb5 axb5 20. Nd4 Qb6 {And black is doing well.}) 18. Nd4 (18. 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.}) 18... Qb6 19. N2f3 bxa4 (19... g6 20. Be3 {And this is very uncomfortable for black.}) 20. Rxa4 a5 $16 {And black's queenside is wrecked.}