It does not really have to do with Catalans. It is a general side effect of mass tourism ("mass" is important here).

In masses, tourist can really become a serious annoyance, mainly because they don't know how to behave (nice example: tourists bathing in some of the fountains in Rome).

There is no wonder, people living in those places are not totally in favor of the tourists. Imagine more than half of the available appartments are used for tourists, and when the season may be 3 to 4 months, the town will be a ghost town for the rest of the time. And "tourism appartments" are more profitable than regular appartments, which means the overall prices go up, making living in those places unaffordable for many people living there. Symptoms like AirBnB do a lot to make the situation worse.

And then, it can be the sheer masses; I believe to remember that I recently read that Dubrovnik now (or is about to) limits the number of cruise ships docking to two per day (down from 5).

I live in a touristy place, which is (fortunately) more aimed at individual tourist, but in season, as it is now, it can be a mess. The lines separating parking spaces are considered to be a guideline for where the middle of the car should be. The supermarkets are filled with "leisure shoppers", blocking the aisles, and so on. Oh, yeah, drivers who almost stop before each and every curve…

Anyways, militant attacks against tourists are not good, and I am convinced that it is only a very small group of people doing that. And, as said, it is not limited to one place.