I'm pretty sure that this is an intended flavor; there's a great and authentic middle eastern restaurant nearby and their hummus tastes noticeably tangy. Others in the Yelp reviews have commented on the same flavor, so it's not a bad batch. It's almost as if there's a fermented ingredient that has the same kind of "tang" as kimchi. Their babaganoush doesn't have this same tang.
Is there a secret fermented ingredient in some hummus recipes that might give off this flavor?