Review bombing is old, though it does seem to be getting worse. Review aggregators that allow user scores/reviews have long been a place where large numbers of people that have axes to grind go to vent their anger. Sometimes they're fans angry about some change they don't approve of. Sometimes they're people with political agendas (which is increasingly the case with review bombing). But regardless of the reasons, review bombing is never about giving an honest evaluation of the overall quality of the work. It is always an act of protest, plain and simple. People who engage in it are simply trying to harm the commercial viability of the work in question. I think it's incredibly immature and counterproductive, but this is the internet and the internet by its nature facilitates that sort of behavior. Whenever I see a work that has decent-to-good reviews getting absolutely bombarded by 1/10 scores, I just roll my eyes and ignore it. It's something that long ago passed the point of being farcical.
The best thing that can be done is for sites to change their policies or simply do away with user reviews. Rotten Tomatoes was essentially forced to change their policies regarding user scores/reviews for films in response to this sort of behavior (though it's still an issue for user scores of TV shows).