Speaking as someone who's been to a lot of protests, ranging from "peaceful as a baby" to "as street medics we treated 1/3-1/2 of the protestors"... the same thing cops are always thinking. As an institution, they don't like the press, oversight, or public scrutiny of their actions. They react the same way to people with press credentials for less prominent organizations, and to regular humans with phone cameras who have just as much a right to record video, the same way all the time.