It's something similar to north/south Italy: they are tired of sharing their monies with the poorer regions of Spain, which they regard as lazy and stupid.
While money is an important factor (obviously) it's framed in completely different manner: former italian Lega Nord independentism is based on feeling/being superior on both the work & money side. And in fact Lega (Nord) was and it's still a right-wing, racist and xenophobic party. OTOH the independentist parties in Catalonia were basically left-wing (and now they cover almost all of the spectrum), and the economic side is more on the "we get +50% of our money out and what comes back cannot even cover the most basic needs". Having said that, there are obviously Catalans on the pro-independence side that can just be Lega Nord supporters. But the funny thing is that there are lots of Spaniards/Catalans against independence that can be Lega Nord supporters as well.
Then imagine how it is for the poor regions of Spain.
Lega Nord supports openly the independence of Catalunya because they are based on the same principles. Sure, there're differences, but the main idea is that they don't want to support poorer regions. Obviously they are not going to tell you that, but that's what originated the conflict.
In fact, some people in Catalunya (obviously a minority) were openly racist with other regions in Spain years ago (e.g., Extremadura, Andalucia), they even made ads in newspapers. Of course, this is not something that's really talked about.