I've witnessed a lot of sour grapes as well. Some reviewers will review books from genres they know they don't like and give it a crappy rating. This grinds my gears. In my mind, these type of reviews are a waste of everyone's time. The perspective consumer and the author. Neither can use that review for anything constructive.
As a book reviewer who reads a shit ton of books, I am bound to run across at least one really crap book. It happens. It puts me as a reviewer in a really tight spot. I know how much blood, sweat and tears goes into just writing a novel. Not to mention the vulnerability of submitting it to the whim of a reviewer.