The Nazi flag was there because somebody was calling Trudeau a nazi. As in: the thing they were there protesting they thought was similar to Nazism, and they think that is a bad thing which is why they are there protesting.

There was as far as I can tell one instance of somebody doing this, and they were immediately denounced and told to stop. I agree, btw. Trudeau is an authoritarian, but he isn't a Nazi.