I started wondering about this myself after stumbling upon a RODAN post and checking out the archive. Did some digging, the most I can figure out is that the people running it did something real bad and got thrown out, which was followed by the archival of the forum and the creation of this one.
Sorry if this is a touchy subject that people don't like to talk about but I figure the dude deserves at least one serious answer.
As far as I understand, this is mostly correct. Most of the old firemods were caught in a private discord server behaving in a way deserving of an on site ban (i don't know the specifics well enough to want to get into them).
This essentially prompted a discussion about what to do, and people wanted to keep the off topic forum around (because it's fun), but rather than just repopulate firebot with new leadership they saw an opportunity to clean a bit of house, rebrand, and start off on the right foot. The idea was to keep the culture around where people could post unique and interesting threads every now and then without being mired in some of the worse elements of the previous culture. Being told to lurk more if you were new while power users were shitposting, some of the edgier humor endemic to some of the banned firemods, that kinda thing. Really as far as I'm concerned they did a good job, between the better older users sticking around to make a good example and an overall deft hand with the rebranding effort.
If it's untoward of me to post this kind of thing because it's in IS then I'm sorry, but the truth is it is kind of important. When I was unbadged, pedos were getting banned from the site and I didn't even know they were pedos until people made jokes on firebot and imo that's kinda fucked up.