Well. How can a highly rated player lose in 21 moves? The other way to ask this would be, how can a highly rated player win in 21 moves? In this video, King explains what went wrong for Giri with the black pieces. It looks like the general rules of developing the pieces and castling the king have been broken. The bishop stayed on f8 for far too long. But, this is because black decided to go for d5xc4, which means that black no longer has a pawn on d5 and thus white can later do e2-e4 grabbing the center. With the pawn on d5, black could answer d5xe4, but now the e4 pawn is dangerous, because it is mobile and can later do e4-e5, chasing away the knight from f6, where the knight is protecting the white squares around the black king. Check it out.