Ok so, as usual, Catalonia's independence movement is steeped in history much like every other independence movement. Regional Spanish history isn't something I'm well versed in, but after looking into it, it's pretty easy to see why Catalonia is wanting the referendum. 

Catalonia has its own unique customs and laws which were given fair respect and autonomy when they were under the rule of the Kingdom of Aragon. But they supported the wrong factions during various wars, namely fighting against the Spanish Habsburg monarchy and revolting various times, only to be put down swiftly. During the War of Spanish Succession, Barcelona was beseiged for 14 months and then conquered, and that led to their laws and customs being heavily suppressed. Not to mention the people of Catalonia are also more heavily taxed than many, if not all other Spanish provinces. So they are essentially trying to gain the Independence they have sought since the 15th century.