I once bought a couple copies of a book as an inside joke for a couple friends.

It was not at all a popular book, I can pretty much guarantee that you've never heard of it or it's writer, and odds are you'd probably hate it if you did ever read it.

I think when I bought them they were going for about $5 a pop.

And immediately after I ordered them the price shot up to like $15

I can only assume that the algorithm assumed that something happened that made that book popular all of a sudden, instead of just one asshole buying a couple copies to give to his asshole friends as a joke.

Took a few months before the price dropped down again.