So here i am looting the corpse of a bear that i just gloriously slaughtered with my trusty two hand hammer, and i hear a commotion just across the waterfall at the edge of Ivarstead, i come in for a closer look and see a purple-clad Ivarstead guard massacring 4 Imperial guards along with a bound prisoner that they were leading earlier on, the Imperial guards did not stand a chance against the Ivarstead's finest, a few swings of the Ivarstead's guards war-hammer and the imperial guards dropped like flies.

Just wondering what prompted the Ivarstead guard to attack the imperial army, or vice versa.....politics..

The Ivarstead guards probably didn't move out of the way fast enough, the Imperial guards will attack you if you don't leave them alone when they are transporting a prisoner. Same with the Thalmor.

Well, Ivarstead is a part of The Rift. Riften is part of the Stormcloak Rebellion.

To them, Imperial Guards are enemies.

Also this. After the Civil war its possible to see the Riften guards attacking the Imperial guards that take over (if the town is under Imperial control).

http://en.wikipedia.org/wiki/Territory_(animal)#Defense.