What happens when another retailer runs the same algorith? One tests the waters with a price hike, the other sees it and matches. The first detects the corresponding increase and maintains the increased price. Rise repeat.

I'm other words, this could be price fixing because making a temporary increase and seeing if competitors will match the increase is sending a signal to competitors that Amazon is willing to increase price if others match. The fact that they revert to a lower price doesn't absolve them from price fixing, it just means the negotiation to collude failed.

The fact that the only factor in play is whether the competitors also increase price makes it blatantly obvious that the increase has nothing to do with supply and demand.