There are a lot of things involved in keeping a game running, and numerous ways for things to stop. Some things are out of ZOS control. Suppose there is a physical failure of a hardware component, or a corruption in a database file? These things can bring a server down. Then there is also things happening on your end. Is your ISP fully functional? Is your power source absolutely stable, or your computer/console could be over-heating or suffering it's own hardware failures. Considering the number of people, the changes made to the code and databases on a daily basis, and the problems with making certain that all platforms perform identically, it is remarkable that the game functions as well as it does.

When they run emergency maintenance, it could be reaction to hardware failure, security violations, or finally getting a serious bug fixed that loud and angry forum posters are screaming about.

Nothing in this world is perfect.