With review sites like Goodreads and all retail sites having customer reviews which are open to anyone regardless of whether the purchase of said item can be verified, the way is clear for trolls to put the boot into any unsuspecting author they deem deserving. I call them trolls because personal attacks on an author can hardly be called reviews. These troll attacks can be the result of many things, but usually it's someone with a grudge for one reason or another. I've seen spates of negative "reviews" appear after a forum debacle or after an author has received a bad press for whatever reason. Self publishing guru John Locke received a raft of these after the revelation that he had paid for some of his reader reviews.
You can easily spot these in the one-star or two-star (if they're feeling generous) section. The giveaway is that they contain no specific details about the book, only vague criticisms of style/genre/cover art spiced with digs at the author. They often lead to bitter rows in the comments too which reinforce the vitriolic motive.
As well as these there also is a subset of trolls who fancy themselves as experts on a particular subject and make it their mission to nitpick every detail in a novel in order to write self-aggrandising reviews which are really nothing to do with the book at all but a means to show off their amazing prowess. Historical fiction is particularly prone to this kind of trolling.
These trolls are more interested in boosting their own self image than relaying useful information. Some barely mention the book, instead listing their own (dubious) qualifications. Sadly their lack of knowledge is even more apparent than the gaffes they are uncovering. Some have trouble stringing a coherent sentence together and it is hard to take someone's pompously proclaimed qualifications seriously when they can't even spell.
So there's much entertainment to be had from troll reviews. What do you do if you get one? Have a good laugh and ignore it. The one thing trolls love is to get attention - it's the reason they do it, after all. Don't feed them.