There are just my opinions based on what I know about what happened from both the news and from friends who were in the area that day.

If the protesters had not crossed the police barricade that was set up to protect the president and they had not started vandelizing buildings, none of this would have happened. The job of the PPB is to protect people and their property, they also had a special job that day of protecting the president. When the protesters crossed the barricades and started destroying property they interfered with that job and they needed to be removed. Also, Portland rioters have a long history of causing millions of dollars in damage. I'm sure most of the people that live in the area remember the May Day riots a few years ago, the New Year's Eve riot two years ago, as well as a handful of other smaller riots in the past couple of years and the cleanup bills that the taxpayers had. It seems for how "peace loving" some of these people are, they sure do enjoy causing trouble and destroying property.