Assuming the ban log is up to date, we appear to be holding steady at around 2 a month. The
Bay12 Posting History thread might have more detailed information if you're curious how that compares to total membership or posting rates or whatever.
Toady tends to delete whatever gets somebody banned, and it seems to often be based on former behavior or even PMs with them, so it's usually not obvious why after the fact. Sometimes it'll get mentioned somewhere by the people who witnessed it, but you could ask around if you were curious about the details or nobody was talking. From what I've heard it's usually pretty obvious to most of the people who have had extensive dealings with the person.
As far as I'm aware, the vast majority of bans take place in political threads, so if you don't take part in those you're mostly safe. Otherwise it's kind of hard to tell
precisely what gets you banned, but at 2/month it probably takes some effort or severely poor luck.
What's causing them to be banned is kiiiinda' stated in the log -- they were doing whatever they were banned for. Not really much else to it.
What's stated in the log is really vague, though. You can get a rough idea, and the low rate implies a certain severity or persistence, but "Bigoted Remarks" is otherwise a pretty fuzzy concept.
As for not getting banned, there's a forum guideline. If you stay within those bounds (and perhaps additionally make sure you're not regularly causing/being heavily involved in problematic arguments/etc.), you'll be fine. Generally, be chill, don't make personal attacks on other forum members, avoid being a bigoted nitwit, don't advocate violence against people on forum or otherwise, don't bring drama to or form other places on the internet, be careful with the lewds, and that's about it.
Note that the stated bounds don't necessarily line up with observed data. The rules specify not to pick fights with other posters or use bigoted language, for example, but "bigoted remarks" shows up fairly frequently, while "fighting" is almost always preceded by "repeated." So... is bigoted language more severe than picking fights? The guidelines don't say so, but the data might imply it. They also don't define either term, which leaves more room for ambiguity.
Hmm, "bigoted nitwit" seems like a hard thing to prove. Many people call each other that.
Well, not calling other people that is probably a good start. Toady and others like to keep this forum relatively chill. It doesn't always work.