A member of an alliance, with at least 100 days in, may challenge the leader for their role. Each alliance member would choose a side and an alliance war would follow. The victor would be leader and the loser would be tossed from the alliance.
Realize that if your alliance is happy then this won’t be a problem. If you have 1 disgruntled member than they would be completely overwhelmed in the alliance war as everyone else would side with the current leader.