It's because of an accumulation of planned reforms, including the one to further privatize the health care system. In addition, people are angry that the government has acted violently against the public. Imagine your entire government doing this to an unarmed population in a city under siege.
It isn't just the 40 people who were murdered, but also the 540 missing persons people who were abducted by the police. Can you imagine the suffering of the mothers of these children and these people? People are wondering what to do. Do they have to stand idly by and watch these 540 people disappear, while the government doesn't want to take responsibility, pull the military out of cities and shed light on the enforced disappearances? We have far exceeded the record of Ayotzinapa, in Mexico, where 44 students disappeared. Now we are talking about 540 people.