Nothing is auto-demained, not trying to sound smart or anything, I'm just trying to clear any confusion. I don't think I've laid out reasons why to demain a thread to the mods. Maybe I said something in the past about it, it's been awhile.
I generally demain a thread if it remains an active topic for some period of time in which most users have moved on from the subject except a select few. Or if someone is just consistently bumping a topic just to get a on the latest list. So there is my very un-scientific approach to using that function. There were probably other scenarios but follow that as a baseline, I still make mistakes.
Shoot a PM to a mod if you think your thread was unmained by mistake. We will likely treat it as a mistake and main the thread and will discuss internally to refine our methods. I remember unmaining Ni and got an earful ( I actually unmained everything in Undead). So in conclusion, PM a mod if it happens, we can quickly work it out and maybe give a red/green bump. No point in causing a stir over something like this. Ideally I would like to have a user customize their own topic feed.