Windows update has a habit of automatically rebooting a machine, especially one set to do a long job. I find this very annoying. Fortunately, on there is a means to stop this. This comes from here.

1. Click on the “Start” button and type

gpedit.msc press Enter.

1. In the Local Group Policy Editor, go to Computer Configuration -> Administrative Templates -> Windows Components -> Windows Update
2. Double-click on No auto-restart with automatic installations of scheduled updates.
3. Select "Enabled", and then click OK.
4. Close the local group policy editor.