After a certain period of inactivity, you will be removed from your current union and transferred to another one the next time you enter the game online.
The feature was made to protect active unions from inactive players, who can hinder the whole union's success. But it applies to server unions only. In a private union you can only be removed by the leader, the server can't exclude you.

Many functions in the game, such as union tasks and duels, require teamwork and a friendly team, so you cannot play outside of a union. If you still want to play on your own for some reason, you can always create your own closed alliance. Other players will be able to get in only with your permission.

Joining a union is automatic once you reach level 9. You can, however, also join a union manually. First, find the button in the upper left-hand corner of the screen with the text message icon. After pushing it, you will see the chat window of your future union. Press the Join Now button, and you’ll be automatically assigned to one of the alliances of island managers.If you don’t play online for an extended period of time, you’ll be automatically transferred to another union.