I thought about leaving TBT last year.
There was a group of users (there still here but they aren't as active together) who were problematic and made the forums really unlikeable, luckily that was only one section of the forums but it was my favourite board, I'm talking about Brewster's Caf? and the Basement. They would always say something rude, always. They never had anything nice to say and overtime I got over them, sadly they still do this and I choose to ignore them. TBT is an outlet for me because people share my interests and that's hard to find where I live, so I'm really grateful to the forums and the people on here because you're someone to talk to when there's nobody else and for the most part the users on here are delightful. That's why I stayed.
There has been several incidents were I've had "flame wars" with people, that didn't make me want to leave but it reminded me who I should avoid in the future, I should thank them for being immature because it has helped me like TBT a bit more.
I was also able to observe this kind of events on the old forums where I was going.
By experience, the "flame wars" and the loss of interest for the forum topic are the main reasons of leaving.
But hey, all amateur forums have undergone this kind of phases. I have read some posts about the loss of activity of this forum. Let me tell you: as a community, you seems very lucky. Many would like to have these attendance and post rates in 2017. You seem to have all the tools at your hands to fully enjoy the next Animal Crossing as a community
When I was part of a moderation team, we often got into the habit of "cleaning up" the forum between two video game releases (Zelda): unnecessary topics, checking forum features and their interest, community brainstorms, etc. We tried several times to better federate the newcomers (more rare than in 2005), insisting that they come to discuss on the chat, and take the time to discuss with them about the past of the forum, and what is still to find over there. Kinda saved the forum.
For my part, I spent more than 11 years on this one. I think that will be my ultimate record. I no longer have the time, with my age and my work, to invest myself as before. But above all, I want to discover new types of forums and communities, help them to evolve, rather than stay anchored too long.