Tannins (and sulfates) are found in all sorts of food. But it's way easier to take the intellectual shortcut and say "it's probably X", instead of "it's probably really complicated and there are a number of factors involved". Some people want simple answers, no matter how complicated the problem is, or how wrong the answer is.
And I didnt meant to narrow - it was the more famous of what you states, and thank you.
My money is on tannins, but that is just me...