You can be automatically expelled from a guild for being idle too long. Next time you launch the game after being kicked, you'll be automatically assigned to a new random guild. The higher the player level, the more they can be idle without any penalties. For instance, a level 10 player will be expelled in one week of idleness, while a level 20 player will be expelled only after two weeks.
Only a guild's leader can expel a member from a player-created guild.