At the risk of becoming a statistic, how is it so many users here are banned? Looking at their posts rarely reveals an obvious reason. And plenty of them seem to be banned very quickly after creation.
Read the rules, man.
Read the rules.
My take is that a huge majority of them have been “sock puppets” (multiple names or UserIDs).
Most of those are just banned without a reason being provided.
Socks, trolls, spammers, namecalling, being a jerk…
Indeed. There seem to be a couple of instant death rules. Threatening legal action, and running (or being) a sock puppet being two. Spammers usually get “disappeared” without trace. Given all they do is post spam there is no issue with removing all trace. But sock puppets may post in a way that makes their posts a part of the narrative, and so they get banned rather than disappeared. The creator of the sock puppet also gets banned at the same time (hence it is an instant death activity.)
But, it seems some people, suddenly get it into their heads to be a total jerk. And engage in jerk like behaviour. Given the absolute ban of sock puppets, and reasonably easy ways of detecting them, it isn’t just jerk-like, but just plain stupid.
Anyway, thread belongs in ATMB, not GQ. Reported for change.
Nearly all of us have been banned at least once.
…don’t forget about all those who have been banned for just asking questions about all the banned people.
Sorry; indeed it does. There I go.
They’re just getting the banned together. Something about paying off back taxes for an orphanage.
Usually the first clue I get that I am reading a zombie thread is seeing the banned posters (is poster a correct word?), then I check the date of the OP. It does surprise me how many once popular posters have gotten the boot, and how many banned posters that one thread can contain. I think some popular posters get the idea at some point that they can say nor do no wrong. I do not think that this a ban happy place, though. I have had no problems - flying waaayy under the radar. 
Moderator Action
Moving thread from General Questions to About This Message Board.
And it would have worked, if…
No one ever talks about things like the weekly mod meeting where they pick 2 members for random banning though. We’re not suppose to know about that.
The first rule of the banning club is that no one talks about the banning club.
Just kidding.
The ones that are banned shortly after creation are often socks or spammers. We probably get half a dozen or so spammers signing up here every day.
Spammers are mostly disappeared, but once in a while a spammer will post a few innocent posts along with their spam (they are trying to blend in and not get caught, which doesn’t work) and sometimes those innocent posts will trigger further legitimate conversations and therefore are left in place.
Posters who have been around for a while and are banned are also usually given a thread in ATMB to explain why they have been banned. Some are banned for socking, some for persistent rules violations. You can usually search ATMB to find the reason.
Yeah, bannings of established, prolific posters don’t happen every day. But the SDMB has been around long enough (more than 15 years) to allow time for the BANNED posters to accumulate, which I think is the main reason it’s so common to run across them when reading old threads.
No, they don’t get banned, they get disappeared…
That and the old threads at SDMB that routinely get bumped are way old. I don’t think there’s any other message board where decade-old threads get bumped all the time.
Two BANNED posters in a three-month-old thread is interesting. Two such posters in a thread from 2003 that has posts from 2005, 2007, 2010, 2011, and 2013 is just statistics.
Nonsense. Such people have never existed. NEVER. It’s impossible to not love Big Brother. And we’ve always been at war with Giraffe Board. 
…the sportos, the motorheads, geeks, sluts, bloods, wastoids, dweebies, dickheads…
Spamming? That’s a bannin’. Socking? That’s a bannin’. Posting in the wrong forum? Oh, you better believe that’s a bannin’.