The Grimtotems may be the most warlike of all the tauren clans, but they're by no means unorganized. If they're the ones who put the torch to the Shady Rest Inn, they'll have had orders from one of the clan elders.
There's a large Grimtotem settlement called Blackhoof Village to the northwest. They've been using it as a base to attack the tower, but I'll bet that you'll find the orders you're looking for there. Check the village's elders for traces of the orders you seek.
Were you able to find any trace of the orders?
<Captain Darill glances at the battle plan.>
I'd say it doesn't get any clearer than this. They've taken it upon themselves to fight back against human settlements in the area and I have little doubt Theramore is next on their list.
- 3900 XP
Grimtotem Battle Plan text:
I have sent you a force of considerable size, elder, but you must take care in your activities. Your first priority will be to halt the advance of the Alliance curs into our lands. Some of the more adventurous humans have already started the construction of a large building on the border with the Barrens.
See to it that they know their kind are not welcome beyond the borders of their wretched Theramore. Lastly, a word of caution to you, elder: do not make your plans known to the ogres of Brackenwall. Those brutes are incapable of even the slightest hint of discretion.
This is part of The Shady Rest Inn quest chain: