Well some of the bans have been for an exploit of Guild Renown system.
A character has a finite amount of renown that can be gathered for a Guild in a [I think] 24 hour period. Once you reach this limit you'll not get any more renown in chest or reward, or very reduced. Not sure. I seemed to have reached that limit over the weekend, when the small guild boost was calculating incorrectly [who was to know].
But if you leave a Guild this renown 'meter' is reset. So if you leave and join a guild it will have reset your renown and you'll be able to farm renown without that false 24 hour limit.
The ease and little consequence of joining a guild means a lot of people join, leave and rejoin a guild over a period of time and sometimes in the one day. Also as a joke some people kick and invite people from their guilds, this has been deemed a renown exploit and thus some mass bans have taken place.
So you can't leave and join, or even be kicked and join a guild too often as it will be deemed a ban worthy event. Of a new system, where the limit of how much you can farm per 24 hours in not known and the mechanics behind any limits or formulas is not known [to players and even GM's], how are players meant to know what and not to do. Specially where the guild system and in/out's have been a common occurrence for many years.
A leader kicks a person from the guild, and that person joins, then is kicked again and again. An officer can effectively cause this person to be banned from the game.