One technique I used whilst levelling my group and a mob got in amongst the pack so that they were behind my main and some of my guys couldn't cast on them was to rotate my main through 180 degrees to face towards the mob and the pack, then press my back arrow key.

(I use this to break follow but the net effect is that my lead character and my pack move backwards away from each other. Leaving the mob castable by all.

It probably wont work to well in battlegrounds, but then again if a warrior or rogue has locked one down the others should move back quicker and everyone able to hit the enemy.