-Migration from one hosting provider to another.
-Hardware failure
-Hardware upgradation
-Datacenter issues where website is hosted.
-New software installations or reinstalling a software which has stopped working.
-Upgrading server operating system or application software.
-Adding new servers to the existing network of servers.
-Changing architecture of the web application.
I have written a few problems but there could be many other which require a complete shutdown of a server or a service(Forum/website)