What events in Ashenvale are you thinking about ?Or the invasion and clear cutting of Ashenvale and attack on Astrannar.
The bombing of a druid school (Alliance doesn't know Garrosh disapproved of the attack).
The invasion of Gilneas.
The complete obliteration of South Shore and the desecration of all residents of the region.
The farmers killed and raised in Andorhal by Horde forces.
Y'know, any of those might be reasons the Alliance would want to stop Garrosh as all these were done as a result of his war machine (and again, the Alliance ((and Horde)) never discovered the truth behind the events in Ashenvale that contributed to the war).