In this game after 32nd move Rc2 of Aronian why black knight moved back ?

In this game after 32nd move Rc2 of Aronian why black knight moved back ?

Looks like a blunder to me, giving away the a7 pawn for nothing.

Black was losing anyway though...

White is threatening Bc4, which will threaten to take the rook on e6 and win the exchange. So, So decides to move the knight in order to free the rook. This results in the loss of the a7 pawn.

Sagar Jha This not only resulted in loss of a7 pawn but also the loss of black bishop..I wonder why black rook not moved to b3 in order to save bishop 

On which move? Do you agree the rook can be trapped by the bishop?

According to Houdini, 32... Nd8 was a huge blunder (for a super GM anyway). It was the only mistake of the match for either player. 

After 32. Rc2, White has a slight to moderate advantage (+0.85), but after 32... Nd8, White has a completely winning position (+2.25). 

I'm sure there was some deeper meaning behind his move Nd8. It's not like he just forgot he was hanging the a7 pawn. Perhaps he miscalculated or overlooked something. 

Sagar Jha, agree that black rook at e3 could have been trapped byu white bishop at Bc4. But on 32nd move black played Ra8 to fork white bishops. But black bishop at b2 is already trapped why black not moved rook to Rb3(instead Ra8) in order to save bishop at b2