From what I can see, in order to get a ban you have to be either an obvious spammer (and the board gets more spam than most people realise because it gets deleted quickly) or a complete pain (in which case you will almost always get a warning first).
However some spammers have been banned in the past by their ip address, the numerical address their computer happened to have at the time. As there are only about four billion of those, they're running out and get reused. If your computer ends up with a banned ip address, your account gets banned. Oops.
So if you have no idea, it is probably an innocent automated mistake.