What might cause a tangy flavor in hummus?
[not a definitive answer]

Making good hummus is non-trivial.  I think roasting sesame seeds is as volatile as roasting coffee beans with a few seconds or degrees changing the flavour drastically.  

It's quite possible the tangy flavour comes from the way they process their sesame seeds.  I've had Israeli hummus (from Jerusalem) and it tasted very different from the stuff you get elsewhere.  Much smoother and more 'settled' flavour, and likely similar to the tang you describe (almost umami).  The ingredients didn't have anything specific listed that could do that.  It also could be the lemons or the zest.

Anecdotally, a local hummus manufacturer told me he gets his sesame seeds from that part of middle east because they lead to better tasting hummus.

Next time you're there, please ask the chef.  most of the time, they'll share something new with you. (I've been curious as well ever since tasting that particular hummus).