My guess would be that Amazon has some algorithm to flag reviewers with odd review patterns. Things like only reviewing products from one manufacturer, or only reviewing new products, or other things like that would potentially indicate a bot.

And so the bot makers have gotten around this by having their bots have a more human-like pattern of reviews. Reviewing a product that already has a lot of reviews makes the bot seem more human.

I was on Amazon to buy the book 'Getting Things Done' by David Allen. And I stumbled onto some really odd reviews that didn't make sense. So I parsed the reviews and did some basic analysis.