This series of posts from David Gaughran has set the cat among the pigeons:
The first one (and most recent) alleges that click farms are using the Kindle Unlimited program (which allows subscribers to borrow books) to manipulate rankings. The specific case being a single book that reached number 1 on 14 July despite very little sales previously.
I’ve not got any particular insights to add other than I think this will be a story to watch. The comment thread is interesting also (one Puppy-adjacent author makes some defence of Amazon).