https://www.polygon.com/2017/10/4/16418832/pubg-firewatch-steam-review-bomb

A review bomb is an act of aggression, a step meant to send a message of extreme discontent with the actions of a particular developer.

The term refers to organized campaigns to give games on Steam low reviews as an act of protest, a situation that has been become so common Valve added histograms to make it easier to see which reviews are likely to be legit and which are part of a likely review bomb. Steam now also separates “recent reviews” from “all reviews” when saying if overall reviews are positive, negative or mixed to make the outliers easier to spot.