People are usually banned for having accumulated too many infractions. Established members are allowed 10 infractions before getting banned. They are unbanned once sufficient infractions have expired to drop them back below the 10 infraction limit. Only a banned member can tell you the actual reason why they have been banned. No-one else will do so other than by speculating. Some offences attract an automatic ban - making a duplicate account or PM spamming come to mind. Check the FAQ/Rules and sticky threads in the Suggestions and Feedback forum for more information.
There are so many possible reasons that it is not really worth guessing. The person who gets banned will be notified by email if they have the option picked to be emailed by Admin. As CB stated, the most common is building up 10 infractions or warnings.