Not out of the blue. "antifa" are anarchists. They may align with some of the values of the greater "black lives matter" movement, but they are not exactly the same. Anarchists want to abolish the police entirely, as well as the courts, the US government, and everything else. Every "protest" they ever go to turns violent, and it has for years, because they believe that violence is the answer. This is nothing new.

Here's a confrontation from a few nights ago in portland. The more colorful woman doesn't think that dumpster fires and violence are productive. The rest of the crowd quickly corrects her. And they always will, because those crowds are not there for peace. That's the same night they lit fires inside of the portland police union.
https://files.catbox.moe/lpuaq9.mp4

Their tactics are very well documented on video all the way back to the battle of Berkeley, when they used rioting and violence to stop Ann Coulter and Milo from speaking. It goes all the way back to the seattle riots in the 90s. Just because you're told they're peaceful, doesn't mean they actually are. They're revolutionaries, revolutions are built on violence.
https://crimethinc.com/2003/11/20/blocs-black-and-otherwise