Ronaldo was dismissed for a shove on Carlos Gurpegi.
Marca says in ref Miguel Ángel Ayza Gámez's notes, he recorded Ronaldo's reaction when leaving the field: "While leaving the field of play, slapped the palm of his hand to his face, while looking at the fourth match official".
This gesture in Spain is often understood to mean "You've got a nerve".
It is recorded that the player was given the red card for "hitting the face of another player (Carlos Gurpegi) with his hand, when the game had been stopped".
Ronaldo now likely faces a two-match suspension.