After seeing many items I’ve been watching steadily creep down in price by a few bucks a day on Amazon, I think I figured out what’s going on.
Walmart is price matching to Amazon, but for some items they have an additional discount if you pick up in store. It looks like Amazon is then price matching the Walmart discounted price. Then Walmart lowers their (pre-discount) price to the new price, and the cycle continues until a product sells out or some other reason.