It's so random it's hard to tell. Like random Blu-ray discs, regular household staples like diapers or shaver blade replacements, etc. And they'll come and go from the exclusive list every couple or weeks or so, it seems. There's a huge long-running thread on Amazon's forum about it.

What amazes me, is given Jeff Bezos' general attitude towards providing excellent customer service, is that Amazon has said nothing about this practice, and it's decidedly anti-consumer. If it was just a Prime discount or something, it wouldn't bother people nearly as much, but Amazon will outright refuse to sell without Prime for a lot of these items, unless it's also being sold by a third party seller you can buy from instead.