Was it something you said? Maybe something you did? If not, you may have been kicked for Inactivity.
If the Alliance is set to "Private", you won't be automatically removed for inactivity.
If the Alliance is set to "Public", you may be removed from the Alliance for inactivity.
How does "activity" work?
Activity is determined by how often you log in and how much you do within the game. It doesn't really take a whole lot. Simply logging in once a day and doing a battle is more than enough to keep "Active" status. Activity also takes into account how consistent you are. If you're regularly playing the game over a long period of time, it actually takes longer for your Activity level to decay, so if you can't play for a few days, no problem! However, if you haven't played the game for long, or play very sporadically, your activity will decay more quickly.