What does Windows 10 use your disk for when you are absent?

I was sitting next to my Windows 10 PC using my phone when my computers disk started to go wild. I proceded to move the mouse and the buzzing stopped. Naturally I decided to investigate and opened my task manager, there I monitored the disk usage and made sure to not touch the keyboard or mouse. after approx. 4 minutes of waiting the disk went from 0% usage to 100% usage and stayed there until I moved the mouse again.

What I then noticed was that two processes specifically stood out from the rest. Namely the "System" and the "Service Host: Disk Defragmenter" processes.

My Questions are:

What is happening in this specific situation, is Windows just defragmenting my computer in the background or is this something else?

The first question is really all I need answered for this situation, but just in case the answer isn't what I expect it to be: Is there need to be concerned if this kind of thing happens? As in: Could a virus potentially infiltrate the System process?

Solution 1:

Windows does a lot of tasks while the computer is idle, as not to slow down the computer when it is being used. It will automatically defragment the disks, download updates, index the files, as well as other possible tasks. Other programs and services like virus scanners might also activate when they think the computer is idle.

Generally speaking, disk activity when the computer is idle is nothing to worry about.