Ok the catalans may be arrogant, but did something significantly change in recent years? Usually these bravado-led movements are a result of stagnant economy like in Italy.
They've been making a huge snowball of this, and it's starting to be too much.
From talking to some catalonian people, I think the last economic crisis, its management, and the corruption scandals in Partido Popular (the party that govered Spain from 2011 to 2018, but is always extremely irrelevant in Catalonia) had a lot to do with it, as it was easy for many to blame those on Madrid, on Spain, or "on the Castilians".
(I'm from Castilla but around half of my family emigrated to Barcelona a generation ago. I don't hate any catalonian who wants independence, they have that right; I'm sick of how both sides use FUD and hatred.)
I understand they are the perfect enemy and have zero sympathy for them, and I agree with you this is the feeling of many people in Catalonia, but I find this argument very weak, taking into account that corruption scandals affected their regional government too (https://es.wikipedia.org/wiki/Caso_3%25).
Besides that, I don't think that there was anything big, it's just with the rise of things like populism and patriotism, it's normal for these movements to rise as well, as they already existed. It was just short of some leaders, and opportunity.
The secession of Catalonia actually appeals to me, but these seems more manipulation by their charismatic leader than an actual resolve. They could do it slowly building the movement and trying to amend the constitution. No rush. Rushing was just trying to get more money which got over their heads as the support and reaction was bigger than expected.
The economy is still fucked. Spain has an overall unemployment rate of roughly 15%, and over 30% unemployment of people under 25.