It's very complicated to explain because it's not a "traditional" movement run by a leader/political party/union. Protesters are all over the political spectrum.

It's the combination of (real and/or perceived):

- rise of inequalities

- new taxes (including the carbon tax)

- removal of taxes impacting only the rich

- sentiment of neglect of the rural areas by the government

- disdain for lower classes by Macron

- lower purchasing power for low/middle class

and probably some other stuff. It's really an aggregation of all that. Add the fact that there is no leader, no precise demands, this makes a nightmare for the government to fix or contain.