A does something to B. (Maybe A shoots B; maybe A refuses to bake a cake for B. It doesn’t signify.)
Social media pick up the story of what A did to B.
Some members of Group Y are outraged at what A did to B, and demand swift retribution.
Some other members of Group Y to are also outraged by what A did to B, but hesitate to commit themselves to a call for swift retribution. They think that perhaps they don’t know all the facts; or they wonder whether the retribution called for might be too extreme. But they stay silent because they don’t want to be called out by, or exiled from, their group.
Some members of Group Z are outraged by the calls for swift retribution. Either because they have a predisposition in favor of A or have bean agitated by the rhetoric of the more vocal members of Group Y, they come to A’s defense, and suggest that what A did isn’t that bad after all.
Some other members of Group Z are also outraged, or at least disturbed, by those calls for swift retribution, but aren’t sure that they can come to A’s defense. They think that perhaps they don’t know all the facts; or they wonder whether the justifications of A are really warranted. But they stay silent because they don’t want to be called out by, or exiled from, their group.
The vocal members of Group Y are even more outraged by the attitudes of the vocal members of Group Z than they were by the original actions of A. They double down on their condemnations of A, and demand that the members of Group Z receive their own retribution for justifying the unjustifiable, defending the indefensible.
The vocal members of Group Z ascend to the condition of righteous wrath. Those who had originally said that A’s actions were wrong but not that wrong now say that A’s actions were completely justified, and in fact could have been much more extreme and still justified. They denounce Group Y’s calls for retribution as “McCarthyite” “witch hunts.”
The vocal members of groups Y and Z make no distinction between the aggressively vocal members of the other group and the silent ones. Any attempts to suggest that the members of the other group are not monolithic and unanimous is met with a sneering hashtag: #notallmembers.
Some of those in each group who had remained silent because of uncertainty or an instinctive desire for moderation realize that they’re being targeted just as aggressively as the most extreme members of their group. They begin to suspect that those extremists were right all along about the other group. In their shock at being so condemned, they tend to forget that there are people in the other group who are feeling just as they feel, for for the same reasons. Their silence has led the rest of the world to think that they don’t exist, and that their entire group of fairly characterized by the behavior of its most extreme members. And gradually that assumption, initially false, comes to be true.