It is a rigging bug on the female human ultra high mesh detail model of the Battlemage Armor Arms. Paradoxically, despite being (from what I can tell) identical, the Battlemaster Armor Arms mesh does not have this bug. Basically, what is happening is that both arm guards are weighted to the right arm bone, instead of the left arm guard being properly weighted to the mirror counterpart in the left arm.
Turn it down to High mesh detail to avoid the problem in play. However, cutscenes will go straight to Ultra regardless, making the bug rear its ugly head again.
Or you can scrape together some Silverite and make the Heavy Armor version of the base armor and then some +Constitution on the arms. That might only be useful for a Knight-Enchanter who would be mixing it up in melee, though...
Alternately, kill and skin some Snoufleurs and see if the Medium Armor version has the bug or not, as that's more likely to have more mage-appropriate secondary material slots in the crafting recipe. Class restriction removal is great when you get the right T3 materials. 