Iker Casillas was whistled loudly after conceding two goals against Valencia off crosses. Right after that happened, the captain was visibly annoyed and shouted back at the fans whistling, as you can see in the video above.
Casillas has been shaky all season long and it's not hard to tell that his teammates don't trust his goalkeeping skills much anymore. The aerial game has always been Casillas' weakest point and he has not improved that throughout the years. Real Madrid have struggled a lot on defending set pieces this season and it might be due to the lack of confidence the goalkeeper is displaying. With Keylor Navas in the bench, it's hard to understand Ancelotti's decision to give Iker the starting spot.
Casillas has been in similar circumstances all season long, and if Madrid finish the season empty handed it's easy to expect changes in the goalkeeper position within the club. Casillas would probably leave the club.