Yeah, for new users/obvious spammers/people who start trouble immediately, the ban hammer comes down pretty quickly. For people who've been around a bit, the threshold is significantly higher.
As CR indicated,
overwhelmingly any regular who's been banned has been warned many times first. I virtually always contact them privately at least once. I do this specifically to avoid calling people out in public, which tends to raise hackles, get egos involved, et cetera. But interestingly some people are
more upset by the private communication; they find it scoldier, I guess, even though to me the exact opposite is true. But whatareyougonnado?
As for the why, it varies, but it's usually just picking fights over and over, particularly the same kinds of fights. Needless confrontation, escalating things unnecessarily over and over, or just skirting the rules. History and intent matter, in that it's possible to get banned for something which wouldn't be a huge deal if it happened once, but when it keeps happening over and over, pretty clearly suggests the person is trying to see what they can get away with. That kinda thing. Mostly these days that means political stuff, trying to pull discussions back into contentious topics over and over.
I try to lay out clear rules when possible, but it's not always possible, and I know from experience that if the rules are
too define, too literal, people always find loopholes even though any common sense observation makes it obvious that they're trying to cause trouble. I leave myself and any mods enough wiggle room to make those judgment calls.
To try to compensate for any ambiguity, I invite questions exactly like this one and appreciate it when people ask me to clarify any edge case stuff. Always happy to do it!