I heard a saying once (I cannot remember the provenance) that could be paraphrased like: "The liberal is someone who is for all movements except the current movement; against all wars except the current war."

There are two important points:

1. Every major movement in history has incorporated elements of violence;
2. Which movements we retroactively consider as violent is determined by sociological consensus.

For example, the American civil rights movement is today considered by people to have been largely non-violent. However at the time the movement's opponents definitely thought of, and portrayed it as a violent enterprise.

Opponents of a movement will always portray that movement as violent. The status-quo consensus perspective on historical protests is written by the victors. Therefore, the hypothesis that "non-violent" protests are more likely to succeed than "violent" ones is self-fulfilling. When protest movements succeed we are less likely to consider them "violent".