What transgressions do people have to commit in order to get banned? I have read the FAQ'so and don't see how the most recent banned member could have done something so terrible to get banned. Maybe something over PM?
I would not want to guess, and even the moderators are unlikely to tell us what happened. It might be worth noting, though, that smaller infractions can pile up, especially if the moderators issue repeated warnings and they are unheeded. So it isn't necessarily one big thing.
Moderator decisions are not up for discussion, particularly in public.
Generally, you're safe if you treat other members of the site with respect. If you do step out of line you'll be contacted by a moderator, who will let you know what the problem is and suggest you stop doing whatever it is you've done. If you continue to push your luck you will be banned. The exception to this is if you're straight out spamming, in which case you'll be banned immediately and without warning.
I have posted this in case you were worried that there were unknown bannination potholes you could fall into without noticing, not as an invitation for a discussion.