The Croatian player got mad because Isco wasn't passing him the ball
Real Madrid played an average game in Anoeta where they won by the minimum margin (0-1). Zidane's squad had few fluency in the ball circulation and, at times, abused with individual attempts.
It is the case of Isco for example who, coming on from the bench, didn't participate in the collective game and went at war on his own. In a moment of the game, the leader of Real Madrid's midfield, Luka Modric, got mad at him and reprimanded him.
As we can see on those images from Cuatro, after Isco lost a ball and Modric made a foul that cost him a yellow card, the Croatian screamed: "Pass the ball, eh, pass the ball a little." Besides, he also told him not to tell him to shut up. Eventually, the two players apologised to each other after the incident on Anoeta's pitch.