The much simpler explanation is just never appealing, is it.... 😱
When you think about how these keyboards are made, what do you think the cycle time is? Half a minute? Probably quite a bit less? These keyboards are almost certainly made on a line that is making a lot more than just Apple Bluetooth keyboards, or at least by employees who are also working other lines / cells. It's most common for products like these to be manufactured in batch runs -- even with lean manufacturing, smaller / simpler / zero configuration / low variety products like these are still typically made in batches.
What that means is that, whenever sales are robust, supplies tend to run low right before the shipment of a new batch. Apple has effective delivery chain management -- they don't flood or oversupply. It's much better to slightly undersupply a consumer product like this than it is to overproduce stock.
What I'm saying is, the most likely explanation is simply that it's a fluctuation in Apple's stock, and that the next batch will be run, just like the previous batch, if it has not already been run, will be shipped in the near future, and availability will return to normal....
