Osasuna went in front through Unai, but Sergio Ramos, after handbags with Fran Mérida, headed on the goal line to make it 2-1 to Madrid. That's where the lighter fell.
Sergio Ramos ran to the corner to celebrate the goal, a usual attitude in all players. The Real Madrid captain, far from being able to go quietly, saw a lighter thrown at him from the stands.
Added to all this is the insults that he received from the same moment he scored a goal and that put Real Madrid in front against Osasuna.
Ramos was also the protagonist in the match after a hard and strong tackle on Rubén García that went totally unnoticed. If VAR had seen it, he could have been sent his marching orders.