Magnus Carlsen failed to attend the Round 8 post game press conference. FIDE regulations state that every player must attend the post game press conference, otherwise he will be penalised by a deduction of 10% of his prize money.
Following the conclusion of the Round 8 game, Karjakin appeared in the Mixed Zone to give brief interviews with the three official media partners to the Championship.
The procedure for players granting interviews in the Mixed Zone was agreed with the players and their management teams at the Technical Meeting prior to the start of the Championship. Both players have granted brief interviews with the three media partners in each of the preceding 7 rounds and several times one player was waiting on the stage until the other one finished his obligations.
After round 8, Magnus Carlsen arrived at the Mixed Zone one minute later than Sergey Karjakin and declined to give any interview. He was then offered to wait for a while in the Mixed Zone or on the press conference stage and Magnus decided to wait on the stage. The World Champion decided to leave the Stage 95 seconds later, even though he was informed by the FIDE Press Officer, Anastasiya Karlovich, that Karjakin was about to come to the press conference. The FIDE Press Officer tried to persuade him and his manager to come back to the press conference room, but Magnus Carlsen declined to do so.
FIDE official statement