Wasn't one of those lesser discussed findings of FB that making people angry makes them more engaged.
I wonder if is a special case of "surprise" - things that are surprising have more information value, and people react to information.
There's also the aspect of something being wrong - if something is wrong, people want to correct it - often angrily if the source advancing the wrong viewpoint is obstinate - and that's engagement.
I wonder if both of those can be teased out. Like, if something is surprising and correct, what's the engagement of that? That probably doesn't drive engagement as much as surprising and wrong.