"A Catalan government that uses public funds to organize a referendum that has been prohibited by Spain Constitutional Court (equivalent of US Supreme Court).
You all have to know that the Constitution of Spain stablishes that Spain is ONE SOVEREIGN PEOPLE and ONE SOVEREIGN NATION; This means that no region can assume the sovereignty of the whole Spain People and organize a referendum to decide their auto-determination. THIS IS THE LAW."
A constitution is a piece of paper. Unless (a) all parties agree to abide by it, or (b) it is backed up by violence, it is worthless. The history books (and present reality in the world) is littered with quasi states that are partially recognised (Nagorno-Karabakh, Kosovo, Israel, Palestine, Taiwan, Transnistria for example), and constitutions that have been violated left right and centre to the point of meaninglessness.
So far Spain had been trying to get all parties to abide by it. As that seems less and less likely to happen, they are bring it out the violence. It is a standard procedure, because if Spain says something is unconstitutional, and people go do it anyway, and there is no punishment, then other people may think they can violate the constitution as well.
It is a tricky situation, because by doing this Spain is turning more Catalans against Madrid, which can well cause problems down the line. However if they did nothing, and the Catalans declare unilateral independence, that may inspire other parts of Spain to the same. The Basques in particular may feel emboldened themselves to demand the same.
The question on my mind is this: If Spain ramps up the violence even more, more arrests, more denials of ability to vote, more suppression of independence opinions or talk, more "police action" against separatists. What will the Catalans do? Will they respond with violence? Roll over and submit? Push even harder for independence? Rarely do states peacefully allow a chunk of territory to leave (because that is the first crack in said states dissolution), so usually there is an armed insurrection at some point.
I also wonder where the Catalans suddenly got the bravery to challenge Spain like this. They have been making noises about independence for decades, but never moved towards it.
Usually when a minority in a country challenges the majority like this, and violates the constitution, it is because they have support of a stronger power. Someone who has an interest in an independence outcome, and the money, global political influence and/or army to support it. Without such a backer Catalonia would be crushed, and the Catalan politicians know this (hence they made noise, but no moves towards it for so long).