YV want to stop revenue going to the gov. That's why toll booths are taken over, speed cameras put out of commission, etc. When YV are protesting they aren't shopping, so VAT (sales tax) revenues shrink.
The violent ones are most likely saboteurs trying to discredit the protests or asshole antifa types or immigrants taking advantage of the situation.
Bank run is being rumored, which aligns with YV stategy; Stop feeding the beast. Violence isn't part of that strategy, but the French gov has to put down the protest by any means necessary before it comes to a bank run. Creating a violence straw man may be a means to do that.