disclaimer: this is largely from memory, so apologies if i get one or two things wrong
Well, back in mid-2009, Redigit, the creator of SMBX and later Terraria, founded the supermariobrothers.org forums, which you'll hear referred to around here as the "original forums". He had released SMBX 1.0 recently and those forums were made to be its official community. Redigit didn't really know how to administrate a forum effectively himself and he hired a whole bunch of moderators to help out - a few of them are folks who you still might recognize like FallingSnow, 4matsy, Quill, and DarkMatt, but a lot of them like Pie-jacker875, RandomGuy27, Mario and Knux (then known as Knuckles96) aren't really part of the community any more, for various reasons. A bunch of other non-staff people have also continued to be part of the community since back then, like me, Joey, Ignoritus (then known as NameUser), Natsu, qig, and Bomber57, among others. The forums looked a lot like these forums, as this site's style is based on the original site.
All was well and good for a while. Redigit maintained regular development updates on the forums and allowed suggestions for upcoming versions as well - a few of these were implemented (Ice Yoshi was implemented through a contest to decide a new Yoshi type) although many of them were pretty dumb as well - but that's what you get with a suggestions forum. In early 2010, Redigit hired two new moderators - Kyasarin and Luminous, who are often (somewhat derisively or at least jokingly) referred to as "the Kathy twins" (the name Kyasarin was found by Kyasarin in mid-2010 to be a transliteration of Katherine into Japanese and Redigit asked to call him Kathy. Additionally, Kyasarin was generally considered the "frontman" of the two). They were far stricter and harsher than any of the moderators who came before, and Redigit put a lot of trust in them due to their experience with forum administration as well as perceived maturity. They are known for banning a very large number of users for offenses often perceived as minor, and for being perceived as having a generally abrasive attitude.
So, the Kathy twins were banning a large number of users around this time, and additionally, Redigit began to enforce COPPA regulations for the first time, which barred any users below the age of 13 from continued membership on the forums. These two events caused a significant cut to the forum's userbase, and many people either left the community altogether or relocated to The Great Empire, a forum administrated by Knux which had originally been created for his SMBX project of the same name. Knux had a very lax approach to administration, and The Great Empire was generally perceived by the SMB.org staff as a haven for banned users.
In mid-2010, amongst almost constant complaints against the Kathy twins' administration and a fragmented userbase, Redigit abruptly closed the forums, handing over the title of "official SMBX forum" to CaptainTrek, an SMBX user with his own pre-existing domain and hosting. This forum, called Trekweb, was administrated by CaptainTrek as well as the Kathy twins, and it quickly developed a rivalry with The Great Empire, which soon became known, almost officially, as simply the Knuxforums (this is generally how you'll see folks refer to it today as well). Trekweb supporters saw the Knuxforums as an anarchic mess populated by trolls, while Knuxforums supporters saw Trekweb as metaphorically hammering in the "no fun allowed" signs. Eventually Trekweb was closed by CaptainTrek due to an unexpected server failure that resulted in a large data loss. The Knuxforums, however, lived on, thanks to being hosted by prophpbb, a free forum hosting site.
Around this time, Knux also started drawing a low-art quality MS Paint webcomic called "Kyasarin's Adventures", in which Kyasarin was depicted as a bumbling ball-shaped character. Although it was created essentially as a shallow trolling attempt, the concept became very popular and Knux renamed it to "The Adventures of Knux", a webcomic which involved depictions virtually all notable SMBX users (also as mostly ball-shaped characters with simple personality traits) and became somewhat central to the community's culture. New episodes continued to be published until early 2015.
So anyways, Knux's forum was basically the only major forum left. Pretty much everyone still interested in the community was forced to hang out there whether they actually supported Knux's forum politics or not. SMBX development itself ceased in late 2010 with the release of 1.3, apparently intended to the final version (whether it was or not, Nintendo called Redigit and ordered SMBX to be taken down soon after). Due to the decreased size of the community, certain people have described this era as feeling like "a family". A few users led by XerX and supermarioman began development of a sequel to SMBX popularly known as New Super Mario Bros. X, or NSMBX. To this end, they created a forum called the NSMBX Forums in late 2011 (September 11, 2011 to be exact), one of the very few older SMBX forums that survives to this day, although it's pretty much dead now. The NSMBX forums were basically a clone of the Knuxforums in every important way, and unsurprisingly shared a large portion of its userbase with Knux's. Even after the NSMBX (later Super Mario Bros. Remastered, SMB:R) project failed, the forums continued to operate due to many people preferring its staff and administration style, which was considered much more structured and stable than the Knuxforums. Ownership of NSMBX was eventually passed from supermarioman to reghrhre after supermarioman left the community.
In early 2013, a group called SORA (the Stop Online Retardedness Act, a parody of the United States bill SOPA) attempted a sort of coup on the Knuxforums, using its connections to the then-current Knuxforums staff to attempt to trick Knux into transferring forum founder (in phpBB forums, this user is the leader of the administrator group any has access to every administrative function) privileges to one of its members, allowing them to take over the forums. Although they were caught by Knux before any real damage could be done, this sequence of events caused tensions to rise between pro-Knux and anti-Knux parties. Later in 2013, the Knuxforums and NSMBX staffs began discussing a possible merger between their two forums as well as Cloud's SMBX forums, another community which had mostly developed on its own, into a new forum called SMBX Revived. This merger was almost successful, and the NSMBX and Cloud forums were closed, but Knux pulled out of the merger at the last moment and continued to administrate his own forums.
Late in 2013, a group called the SMBX Council was formed, consisting of Knux, Valtteri (who had served as an administrator on the Knuxforums and SMBX Revived in the past), Quill (a longtime community member famous for taking years to finish his episode), Joey (the guy who runs this forum and a longtime prominent community member), reghrhre (former leader of NSMBX), and several others. This group's intent was to create another new forum via a merger between SMBX Revived and Knuxforums, this time operated by Joey on his own servers. This was significant because, since the closure of Trekweb, all SMBX forums had been hosted using free forum hosting sites like prophpbb, which allow only limited customization. Again, the merge appeared successful, but Knux pulled out of the merger at the last moment and deleted all the posts that had been made in the few days since the forum had opened. This time, probably due to Knux's more dramatic actions and the popularity of Joey's new forum, his forums' activity dropped significantly for the first time in years, and he eventually closed his 4 and a half year old forums in August 2014.
And, well, here we are. Man, I love to reminisce! Not that this community isn't still a lot of fun. I could describe the history of this forum too, since we're two years old now and the biggest SMBX forum since the original forums at least, but that'd take another 4 or 5 paragraphs. Hope this helps you understand our community's rich history!
edit: Joey himself wrote a similar thing in the past,
The Story of SMBX, if you want to check that out. He has some more details based on IRC interactions and stuff, which I've rarely been a part of, though mine has info that he lacks too, based on the time of writing.