When the night elves first joined the Alliance, it was after many centuries of seclusion. In Season of Discovery, it's revealed that not all night elves accepted the decision to ally with other races. A rebellion led by Illari Duskfeather tried to keep Kalimdor for the kaldorei.
The questline begins with the discovery of a pillaged campsite in Desolace, where you find the grisly remains of two humans and a dwarf. Based on the markings on the bones, the attack was a savage one — but not done by animals or centaurs. We learn that these attacks have become quite common lately — since the victims are always humans and dwarves, it's assumed that a group of Horde marauders are behind them.
To our surprise, we learn that the group behind the attacks is led by a night elf named Illari Duskfeather. While she attacks Horde on sight, Alliance players can question Illari further about her reasons behind the attacks.
As it turns out, this night elf hasn't accepted her people's place in the Alliance. As far as she's concerned, humans and dwarves are just as destructive as any Horde race, and Kalimdor should belong to the kaldorei.
Chillingly, Illari even subtly predicts that an alliance with the other races will eventually lead to the burning of Teldrassil — she speaks of humans at the foot of Teldrassil, torches in hands.