They could be upgrading hosting infrastructure - sometimes this requires servers to be shut down or restarted. They might also be applying database changes such as migrating data from one server to another, or updating the structure of the database to improve performance or support new features.

Honestly, there are quite a number of reasons for planned downtime.

Unplanned downtime is a different story. Usually that’s because something unexpected went wrong and there will be engineers trying to get things back up and running ASAP.