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.
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