In my recent case I believe it was because of a weak bluetooth signal caused by external factors, but I would have to see your setup to speculate what might have been happening in your specific situation and even then I might not have a clue. 🤷‍♂️ High on the list of things I would look for are sources of RF interference such as florescent lights and other Bluetooth devices or anything that might attenuate the Bluetooth signal such as the steel base of my monitor. Sometimes moving a device a few inches or changing its orientation is enough.

In my experience, temporarily plugging in the device and then unplugging it has reliably re-established a bluetooth connection that has previously been reliable. The "nuke and pave" option to "un-pair" the device (System Preferences > Bluetooth, select the device and click on the x at the right end of the selection bar) and then have it re-discovered and re-paired (it may take a while for the computer to "see" the un-paired device) is frequently recommended but plugging in is a lot faster.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein