When you cancel a Time Machine backup, it takes some time to stop:
Actions of programs have no inertia, they can be stopped immediately. If stopping something would leave a system in an inconsistent state, and you need to clean things up first, that’s what the user interface should say. But “Stopping” makes no sense. Just stop.
This problem is not uncommon, and I was thinking about writing a post about it for some time. But here is what happened recently. Mail in Yosemite Beta had already been “Disabling” my account by “Closing” something for several minutes, so I decided to cancel it — and got this:
I had to force quit it after a couple of minutes.
Added in August, 2016: And here, Dropbox is “Pausing”: