Check this out... from wowwiki
"According to the Cairne Bloodhoof article, the Grimtotems have been slowly poisoning Cairne, causing him to finally perish following an honorable duel with Garrosh Hellscream; enraged, Hellscream responds to this treason by leading an invasion of Thunder Bluff to force the Grimtotems out.
In the current Beta build, there is a quest where the Alliance provides the Grimtotems with weapons to fight against the Horde. It is at this point that the Grimtotem them self say they will align with the Alliance."
http://www.wowwiki.com/Grimtotem_tribe