It could be one thing or a combination of things that give it that cheesy flavor. It could be a natural anti clumping ingredient, corn starch or MSG (which I "think" HC has in it). It could also be a very common ingredient that is ground finer than you're used to and it gives it a different texture and flavor.
I haven't had HC in a couple years, but from what I remember it is very good but I don't recall a cheesy flavor.