Reviews sometimes return, btw. It has been mentioned here.
After having almost my entire backlist of romance novel reviews going back ten years removed, I really looked into this. I think after my experiences and the experiences of my close friends, author friends, and students that I teach about publishing (I teach on a college campus via SCBWI), I have it down.
First, an autobot makes some sort of match on your book and triggers an audit. Usually it's one of two matches:
1.
Gift card match. You gave a gift card to someone, say a fan who won it during a promotion, and that fan wrote a review of your book later. It could also have been a gift card to a friend or family member who later wrote a review. It reads to the autobots as a payment for a review. They nix the review and it triggers an audit of your reviews.
or
2.
IP address match. Some are simple. Your sister comes to visit and makes some purchases on Amazon while at your house. Now her Amazon account has been associated with your home IP address, where you upload your books. A year later she tries to be helpful and writes a review of your book. It reads to the autobots that she is family. (Same thing if you go visit her and log in at her house.) This triggers an audit.
The IP address gets worse if like me, you teach in a big public environment and log into KDP to show students how to navigate the reports. Of COURSE your students immediately log into KDP to follow along on their own reports. Now we're all one big incestuous family. Even if the student later thinks--hey, I'm going to read one of the books by this teacher, and reviews it, it reads as family/friends to the auto bought. And an audit is triggered.
The audit goes much deeper, and starts catching reviews by other authors, and you may not even know those reviewers were also authors. It will also catch if you have purposefully or inadvertently reviewed someone who has also reviewed you. I'm actually sort of sad when an author I love reviews me, as I know I can never, ever review them back. This happens on KB regularly--I love you guys, but I have to love you from afar once you've touched my forbidden fruit review page
When my account was audited, I got super screwed, because since I teach, tons of people I didn't really know, who had just been to book events or heard me teach, got reviews deleted--even old ones, from before they met me. On what I call "Doomsday," about 100 of my own reviews on Amazon, and about 50 reviews of my works were deleted.
I caught it in the middle of it happening, and quickly printed out what I had left. When more were taken down, I examined closely some of them -- far flung locations that couldn't have met me or shared an IP. I can only guess that they are also authors in my genre under another name, accounts Amazon has already associated.
I would not say my sales were harmed by this because truly, sales rise about 20% for me every month, but a couple books had to build reviews back up before I could do promos with them again. Jinnie Wishmaker was the hardest hit because so many SCBWI members from class had bought it and reviewed it, which I had not asked for or encouraged, but I guess they wanted to check it out. It went from 16 reviews to 2 and has not recovered.
My advice if you love a book by another author and want to review it: offer a blurb review instead. If the author wants to use it, they can put it in the editorial section as an endorsement. Many KBers could use editorial endorsements in that section of their pages. Even if customers think they are not affected by those, their eyes still skim it and it makes an impression as they scroll. My bestselling books are always the ones with editorial reviews, as it just makes the page look more pro.