An Ancient Enemy

They are on TOP of the cave for anyone wondering. Just talk to the guy and it will start the event. Sometimes she will draw power from the fires, just right click them. If the event is already in progress, you will still get credit for it if you help.

Do not attack Zar'jira. She will one shot kill you, even at level 5 or 6. Just sit there and listen to the event between her and Vol'jin and kill the level 4 adds and stomp on the braziers when prompted.

I had the same problem but I came back later and could launch the small event by asking Vol'jin.
Here is his dialogue with Thrall :

* Talking to the player *

Vol'jin: I've no love for Garrosh, dat much is for sure, but leavin' tha Horde is not a decision I be takin' lightly.

Vol'jin: Dere's only one with tha answers I seek. Ya can stay and watch if ya like.

* Thrall arrives *

Thrall: Vol'jin. It is good to see you, brother.

Vol'jin: Thrall! I am glad ya be well. Dere were rumors otherwise.

Thrall: Indeed. Someone did try to kill me, but that is not my greatest concert at the moment. The world itself calls for my aid.

Vol'jin: I must beg ya council, my friend. I can't be standin' by Garrosh while he be turnin' our people against each other for the sake of war. My respect for ya does not extend to dis new Horde... I am thinkin' of leadin' my people away.

Thrall: Vol'jin, I chose Garrosh because he has the strength to lead our people through these trying times.

Thrall: For all my supposed wisdom, there have been moments that I've barely been able to hold the Horde together. The Wrath Gate and Undercity displayed that clearly.

Thrall: The Horde cries for a hero of old. An orc of true blood that will bow to no human and bear no betrayal. A warrior that will make our people proud again. Garrosh can be that hero.

Thrall: I did not make this decision lightly, Vol'jin. I know our alliances will suffer from it. I know the Horde will be irreversibly changed. But I made this choice with confidence that Garrosh is exactly what the Horde needs.

Thrall: I'm trusting you and the other leaders to not let this divide our people. You are stronger than that.

Vol'jin: I understand, brotha, I will tink on this and be troublin' ya no furtha. You have a world to be savin'.

Thrall: Throm'ka, old friend.

* Thrall leaves. Vol'jin is now talking to the player *

Vol'jin: Ya are strong and proud, youngsblood. The Darskpear will be honored ta have you fight beside d'em.

Vol'jin: Thrall's word are true, as dey always be. The Horde is much more den a few old stubborn leaders and a handful of heroes from Northrend. The people be cryin' Garrosh's name... at least for now.

Vol'jin: Still I be hopin' Thrall will return to us one day. That future right now be lookin' very grim and bloody.

Vol'jin: Go now. Make tha Darkspear proud. Dere are many wars ahead of us. I'm sure ya be havin' a part to play in all of dem.