what's going on?

Windows is a complex system, any complex system requires regular maintenance, As this maintenance takes resources from the pool available to the user it is set to happen when the system is idle.

You can see the processes that are active at any time by opening the task manager, unfortunately a lot of these "housekeeping" processes will be shown as Service Host followed by a descriptive text, if you wish to investigate a particular process, (eg. One taking a lot of CPU, Memory or Both), you can right click on the Service Host entry in Task manager and Select the Search Online option

Basically these processes you see are needed to keep the system running efficiently and are designed to quickly end when any user action needs the resources used so are not an issue. There are occasions where something goes wrong and a process can use more resource than usual and this could be a sign of a fault or even Malware, so it is useful if you can spare the time to get used to what runs and what resources are in use on your unique system