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? 
The recipes I've seen include both lemon juice as yogurt. I suppose the tangy flavor comes from the lemon. 