I’m sure people are lining up to argue that this (and the violent anti-Japan protests that occurred throughout China this past week) are good reasons why Chinese authorities normally do not allow demonstrations, but I think it’s more likely that people are pushed to desperation by a pervasive sense of hopelessness in their ability to affect change.
Two separate forces lead to such violent protests: a feeling that nonviolent expression will not solve anything (supported by the inability of petitioners to gain audiences, activists to change laws, and journalists to report on censored topics) and that violence can solve everything (supported by policies of forced eviction, forced abortion in the implementation of the One Child Policy, and the stability maintenance system).