Cairne was killed in a duel against Garrosh over a disagreement.
Tauren intro video confirms: http://www.youtube.com/watch?v=hmXZqwbKb60
EDIT: Apparently this is old news. The reason for his death had been disputed as far as I knew. Sorry for reposting, then.
P.S. Trolls, go away, we don't like you.