amazon effect
Nasty competitors will try to one star your book and trash your rep.
Here is a sample review.
"I'll admit: I do not own this piece of garbage. It'd be a waste of trees to purchase it."
This is on the book Halo Effect.
You can see the effect a few one stars have on sales, it is hard to imagine. Often a brand new book will crash after attacks from the swarm of bad reviews.