I have no clue what the cause is, but I've seen it before, some effect that fully restores armor. Watch the video again and look for four shields made of light surrounding him.
And if his HP is staying about the same, it could be that his Shock rank and the monster's damage sync up fairly nicely.
Still, it's pretty hard to get skill data on Karok. (And I'm pretty sure most of the AP costs I have are wrong.)