How to "spike" an otherwise peaceful rally. Was a similar technique also used to turn the protests in our cities into violent riots?

Sen. Johnson reads excerpts from the eyewitness testimony of J. Michael Waller:
https://centerforsecuritypolicy.org/author/imp_jmichael_waller/

Pay particular attention to this long video from 20:00 - 24:25.

This hearing contains testimony from several and a wide range of opinions. But the transition from a happy, friendly, peaceful rally that was respectful of the cops into a riot is very instructive.