It only lasts until he attacks, so it doesn't appear to be an effective shield as I would first guess. I had 8-10 paladin guardians at the time, and only 1 ever had this happen.
What is this shield/aura the paladin has around him? What does it do?
EDIT: The paladin also had this aura active between waves, so I don't think it is related to any kind of enemy. Proof: the "2" in the screenshot below is the countdown to the next wave.