As someone living in a middle/lower class neighbourhood, I would say that there's been some chaos. I myself saw two train stations being burned and a supermarket being looted in front of me. However, as someone who's been to these peaceful protests since October 18, I think that the treatment the government and police have given to these protests is one of the big catalysts for the chaos and violence. Peaceful payment evasion protests started on October 14 and the government just decided to treat protestors as criminals, sending police's special forces units to train stations, where violent encounters between officers and protestors (at this point most protestors were 13-17 years old as the movement started with secondary students) which would infuriate people and on Thursday of that same week protestors started to destroy charging devices and opened gates to closed stations. From that point violence only would escalate.

I think that both police and military forces have shown a disproportionately violent response.

Last INDH (National Institute of Human Rights) report [1] says there have been 3193 detainees, 1092 injured in hospitals, and 88 lawsuits, of which 17 are for sexual harassment (including rape).

It might sound "reasonable" for some to declare emergency state and send military forces to the streets, but it's the first time that something like this happened since the dictatorship. Police officers are throwing teargas to kids and the elderly in the protests. There's also a big number of injuries that will never get to be informed.

About the reasons, I would say inequality. The increase in train fares is nothing more than a detonator. There's no organization leading the movement, so the demands are scattered (pensions, health, social contract, neoliberalism, concessioned highways, etc).

[1] https://twitter.com/inddhh/status/1188525375577645057