The triggering event, as I understand it, is they did not censor the President of the United States who said something to the effect of riots lead to death. There's two ways to interpret this. One way is that eventually people will get hurt in some way if riots continue. Another way to interpret it was as a death threat to protesters. FB's position was that even if it was a threat, that the protesters needed to know that the President was going to send in the military to come kill them. So they did not censor or otherwise flag the post. This is perceived as FB promoting violence.