Catalonia and Basque country are the most developed places in Spain, from an industrial point of view.
Historically there were the biggest supporters of Spain, specially in the times of the Empire.
For example Cuba was controlled mostly by Catalans.
After losing Cuba and Philippines there were lots of social unrest in Spain, and specially Catalonia. Lots of people moving into cities. It was the time of romanticism, nationalism, communism and anarchism.
Catalonia had lots of anarchist combined with a rich bourgeoisie that had repatriated a fair amount of the wealth from Cuba.
The rich bourgeoisie wanted more autonomy so they could be richer and not share their wealth with less developed parts of Spain,(while preserving the market to continue selling to them) like Andalusia and Extremadura, that were extremely poor at the start of 20th century.
In 1934 they supported a secessionist movement to declare independence of Spain trying to take advantage of the communist movement to declare Spain a dictatorship of the proletariat that only got traction in the north of Spain, in Asturias. Those that took part of the movement went to jail.
In 1936 the left coalition of Frente popular, that included communist and anarchist wins the elections, and make an amnesty freeing all the people from the 1934 coup. They start burning churches, raping nuns, and killing people (more than 700 in over 7000 incidents). The gobertment does nothing, because they are sympathetic to them, or they just want revolution coming(like Largo Caballero).
That leads to the revolt of the army and the start of the Spanish civil war. In Catalonia the supporter of the secessionist movement in 1934, Companys, is in charge.
This led to terror in Catalonia, in which the religious people were exterminated, something that Companys was specially proud of and made the bourgeoisie specially afraid as the anarchists went after them.
This made bourgeoisie later one of the biggest supporters of Franco, the irony.
After Franco won the war there was a very centralized State in Spain.
When Franco died, they decided to give autonomy to all the provinces in order to appease the nationalist(they did not supported Independence openly at the time).
In Catalonia secessionist were a minority at the time. Most people wanted more autonomy(who doesn't). The man who controlled Catalonia, an extremely corrupt person called Pujol was very secessionist and started creating a network of secessionist in all local public institutions.
At the time the Spanish gobertment of Felipe Gonzalez was also very corrupt, and the King of Spain(that pocketed deals commissions of energy deals with Saudi Arabia). They knew each other and made sure that Justice did not act upon them.
Controlling the education, local TV, and most of the economic system in Catalonia make more people become secessionist, specially young people.
Now it has become clear that Pujol was a tremendous corrupt person and Justice went after them, the guys that pocketed from 3% to 10% of all public money in Catalonia for decades.
Catalonia and Andalucia had been extremely corrupt for a long time.
That(most people getting to know the corruption) destroyed the nationalist party in the elections and made them what we call "flee fordward" with independence, even when there are not enough people yet for it to success without a doubt.
If they get independence, with their own justice system, they will be able to continue doing what they have been doing for a long time.
Right now most supporters of it are kids, 14, 16 years old and people under 30 years old.
They had been promised that they will be like Switzerland rich, part of the EU of course, with all the advantages they have today for being part of Spain(like exporting 70% of their products there)without all the disadvantages.
The EU has said that an independent Catalonia will not enter the EU.