I recognize that this isn't a problem that will normally occur, because I was running it with my level 85 mage, and with a standard 5-man group the problem propaply won't happen.
Anyways. I started the battle with the guy where you need to pick up a shield to protect yourself from the archers, otherwise you'd get knocked back. Most likely you need to do this even as a level 85-90, cus you'll still get knocked back. But before I picked up the shield, I got close enough to put living bomb (a DoT) on the boss. But when I reached his dead body, my character wouldn't drop the shield.
Thereby I was forced to walk (very, very, veeery slowly because of carrying the shield) out of the instance.
I therefore suggest that you add a mechanic where you have the opportunity to drop the shield whenever you want ;)
Sorry for the long story :P