In most cases, bans are issued to punish players who use bots. Bots are detrimental to the game and its players, and are strictly forbidden. The punishment for using bots is a permanent ban on the account.
Other common reasons for bans include behaviour and/or use of language that break the rules of conduct (insults, racist/sexual comments, etc.), and buying or selling accounts, kamas or subscription time to/from other players. These infractions can result in bans that range from a few days to permanent bans (depending on how serious they are).
Of course, this list is not exhaustive, but it does summarise the most frequent reasons for bans. If you still do not understand why you have been banned, please contact Support by clicking on “Submit a request” and then selecting the “sanctions and infractions” form.