Spain's constitutional guarantee of sovereignty also guarantees the distribution of power to semi-autonomous communities in Spain. The 2017 referendum withdrew the rights of Catalonia. Many commentators have pointed to the implementation of Article 155, the so-called nuclear option which withdrew regional powers from Catalonia as an authoritarian overreaction to the issue of Catalan independence. The implementation of the ruling temporarily placed the institutions of state back in the hands of the central government and paralyzed Catalonia. 
Spain's reaction to the illegal referendum has ironically given Catalonia legal grounds to make a bid for independence as Spain has attempted to punish Catalonia as a group, just as it did under Franco.
Many argue that if Catalans had not enacted an illegal referendum, they would never have had one at all, as the Spanish government has no intention of listening to the separatists and granting them a legal vote. The Spanish central government's reaction to a bid for independence has been to throw Catalan leaders in jail. Catalonia is asking for a dialogue, but Spain does not want to negotiate.
Madrid has proved it is willing to be authoritarian in its dealings with Catalonia. The separatists are trying to get their voices heard.