Aronian played two sub-optimal moves in the middlegame - 16...Re7 and 18...h6. At each of these moves, the better move as per analysis by Houdini (depth 25 and above) was Nd5! (Houdini even prefers 17...Nd5, but 17...Rfe8 is not bad)
So his "mistake" was missing the opportunity to play Nd5, and instead playing 16...Re7 (which seems to be the major culprit) ...
I am not even close to a decent level in chess, therefore you have to be really skeptical to what I am going to write here. But here is my ~2000 rated opinion:
Black is not winning after 19.f4, is he?
This is kind of funny to state that black is winning here. This is just 19-th move (so people just went out of the opening), which means that the game just ...
According to Anish Giri in New in Chess 2014/3, 13...Nf6 was a dubious move. Black should have played 13... Nf4! 14. Nf3 Nxg2! 15. Kxg2 a5!! with a dangerous attack. The threats are 16... a4 and 16... Ra6.
Anish Giri also criticizes 12. Nxe5 from Anand.
[Event "FIDE Candidates 2014"]
[Site "Khanty-Mansiysk RUS"]