Another reason I might stop watching someone is if they simply aren't producing anything that interests me anymore.

I've un-watched someone for just being plain annoying and getting into a lot of drama.

Much of the above

Usually it's just due to inactivity/banning, but more than a few times I've stopped watching someone because their style didn't change at all. They're a good artist, but they don't improve or everything they produce looks the same. It's pretty much like watching TV reruns. Fun for a while, but in the long term unbearably boring.

Sometimes I'll go through my watch list and delete someone because I've changed what I like, and I no longer am interested in their style or themes
That's pretty much the only time I'll delete someone

I think I've never unwatched anyone, but maybe I should, so that I actually get time to look at their work. I'm a "few" months behind on my watchers deviations, and I feel like I'll never ever catch up again

If I discover that they have cruddy personalities, sometimes I decide they aren't worth supporting. Otherwise, it is mostly for style change.

i unwatch someone is they're an ex bf of mine.

When I thought their was good but then I realized, maybe years or months or weeks or even days later, it really wasn't. xD;; I'm cruel..