This started because of a raise in bus fares, but the fact that more and more people are protesting has a lot to do with police brutality in repressing the first protests.
Most people in Brazil don't usually protest (nor actually fill complains when wronged), due to a variety of reasons.
But whenever there is a protest it is quickly classified into either: a) "just vandalism", or b) "peaceful protests".
For what the media/elite calls as 'a)', the typical Brazilian attitude is quite weird. Most people in Brazil have this very fascist view that police should be brutal with "criminals". Whoever they might be. As long as not themselves.
However, for the rarer cases where 'b)' happens (and is viewed as such by most people), which was the case here, it might be different. In the current case, police brutality backfired and became one of the main reasons for the escalation of protests and its spread to other cities.
PS: I'm a Brazilian citizen.