I'd have to research to get the actual times and events correct, but it went something like this:
For 5+ years, e-mails from the forum would not send out to Microsoft-owned addresses. This drove people like Andreas Miko away since it ruined their usage of the forum.
A lot of people were scared off more recently in early 2016 when Chrome (and Chrome only) started complaining about the forum not having an SSL certificate (this is Google's way of bullying the entire Internet onto SSL and a lot of people are rightly upset about it).
Then there was actual malicious code inserted into the forum software (it was pretty benign, and just redirected people who came to the forum from Google searches to scam sites) which scared off a lot of people in mid 2016 due to browsers warning about it (mostly Chrome). Australian ISP's even went so far as blocking access to the site entirely because of this. Australian members were dead in the water.
Work on a solution to these problems started in earnest in mid-2016.
A good bit later, in mid October of 2017, the forum went offline completely due to unknown host/server issues.
The forum stayed offline for about a month. Most people had written the forum off by this point as completely dead and never coming back. They moved on.
These were dark days.
As mentioned, in mid-2016 when Chrome started complaining about the lack of SSL certificate, I started working with Crim on a plan to fix that. When the malicious code was discovered, the plan changed a bit and went into over-drive.
The plan ended up being that I would re-write the forum software from scratch. I can't stress enough how many problems this would fix (including the e-mail issue). I also can't stress enough how much work this would be. Imagine writing all of Facebook (circa 2007) by yourself, in eight months.
The forum software had bit-rotted so much it was not salvageable. We had dozens of bugs listed in the moderator forum that I wanted to fix. Not to mention VBulletin is a huge resource hog, and would cost way too much to keep hosting, and renewing.
Anyway, I started writing my own forum software in early 2017. When the forum went offline in the middle of October 2017, by then I had spent every waking moment of 2017 working on the new forum, but it wasn't ready for prime time.
It didn't matter. Something had to be done. So I rushed my forum software into production in early November, 2017. The forum came back online, but it was missing a lot of features. It still is, but back then you could create new threads, make posts, send/receive PMs, and go into the chat room. But that was about it. Editing posts didn't work. Emoji didn't work. BBC tags didn't work. URL auto-linking didn't work. Password resets didn't work. The ever important
New Posts page didn't exist yet. Tons of stuff was lacking.
I spent 2018 implementing more and more features we're used to having. However, because of the stress of writing the new forum in 2017, I got very sick toward the end of 2017 and had to slow down a good bit in 2018. Still, tons of stuff got done. This entire time-line is cataloged in this thread:
Major Forum Changes Ahead. I am still updating that thread as new features are implemented. I highly recommend you go to that thread and read it. It is a fascinating read, and I would appreciate it if you did.
Because of the issues of 2016 and 2017, the forum community has scattered. Some never went anywhere and are still here, and more are coming back each day as they learn that the forum is back up (even though it's been back for nearly a year now), or they learn that the forum really is the best place to be.