The rod holding mechanisms require active power to hold the rods out of the reactor. It sounds like once in a while, a rod accidentally drops, which is infinitely preferable to the alternative, of once in a while a rod failing to drop. But that unplanned adjustment to reactivity means, you don't want to simply use other means to boost the reactivity back up (like reducing boric acid in the water), because once you get the rod back out, it'll be too reactive. So they insert a couple more rods and call it a shutdown to take a few days or weeks to investigate and perform maintenance. Sounds like this is a highly planned for and fairly routine procedure, if not ideal. I'm sure they'll find a couple reasons why the rod unexpectedly dropped, and make changes to prevent an identical incident in the future.

The rods prevent an emergency. They’re a fail-safe.

It’s possible nothing was wrong and some sort of “blip” triggered them when they weren’t needed. And if so that’s annoying for the operators, but nothing bad was happening. And it confirmed the failsafe works.

If anything was starting to happen and the rods dropped and immediately stopped it that’s a failsafe working perfectly.

Either way this is fine. I know nuclear + unexpected = scary for many (maybe not you), but I don’t think that’s necessary here.

Sounds like the Nuclear Regulatory Commission explicitly categorized it as a "non-emergency"