To me, something like Windows 8 happens when a large company who has ruled their market for many, many years - loses the basis for their existence.
The base for any company's existence is meeting their customers' needs with the product they supply. Let's look at this. Microsoft has - since 1995 - used a computing paradigm of Desktop, Start Menu, Applications, and Control Panel (to manage settings). So what did they do for Windows 8?
Everything I've mentioned is still there - but under a layer of tiles and a different screen shown when the computer starts. Microsoft chose to change it's users' interface far more radically than when it changed from Windows 3.1 to Windows 95 - and when it had a far larger user base.
Microsoft said it wanted to unify its code base across all platforms (mobile, tablet/laptop, and desktop) to make it easier for developers. Very nice.
ARE DEVELOPERS THE CUSTOMER? No, they are not. In this case Microsoft decided that developers were their customer - WRONG.
A touch interface works for mobile (phones) - and tablets. Maybe even for laptop computers. But I DON'T WANT TO SWIPE MY OILY AND DIRTY FINGERS ACROSS MY DESKTOP COMPUTER'S MONITOR. Doing that makes it harder to view/read my monitor and I've been fighting dirty monitors since I went to computer school in 2000. Why do I want to have a dirty monitor - even if it is a touch enabled monitor - when it interferes with my ability to read what's on it?
There you go.