What does it mean? I don't get why the socialists are upset. Don't they run the military and police now?
The dominant political event in Spain was the civil war, which basically was between the Republicans or Socialists (read Communists) on one side and the Catholics on the other. This goes back all the way to the time of the Marxist Leninists in Russia in 1917.
Spanish Socialists have a history of violence, which included the murder of nuns and priests before they were defeated by Franco.
So there is a good deal of hatred between the Socialist government and the descendants of murdered Catholics. After Franco left the scene, the Socialists thought they were owed their own term in power, but for the most part the voters have thought differently.
I have an aged friend in Spain, originally a friend of my Aunt, some of whose relatives were murdered by the Communists. I believe that for the most part Catholics have been forgiving of the past but Socialists have not.