It's the typical second biggest city not having internal relevance compared to the capital even though it's as good, together with the common claim of everything happens in the capital, decisions are there, most money stays there, etc.
Also, it's slightly different culture, language, etc. And around WW2 time, there was an actual civil war.
Discontent keeps happening, catalonian patriots gets in power, central government remembers that constitution prohibits secession, they do a referendum anyway, (90% win for getting out), get arrested/exiled, with draws bigger attention to the cause, Streisand effect and all that.
Spain issn't really an united nation, you have actual separatist terrorists in Basque (which have a peace treaty recently), the new separatists of Catalonia, Andaluzia has seen the rise of dissidents as well. Add to the fact that some regions actually have their own language.
I remember one time while crossing the border by car, seeing a tourist information sign vandalized because in was written in Castillan (language of the capital, main "spanish" language). Beneath all the vandalous paint, was a sentence: "speak the <local-language>". And this was not in one of those more dissindent regions. Later I got into the city, and every sign was in multiple languages