SHA256 IVs / NUMS numbers I've read that if the IVs for SHA256 were arbitrary and / or unknown (secret) than a backdoor to predict outputs would be possible.

How do the values chosen for the IVs / publishing the values prevent such a backdoor and if the values were arbitrary / secret, how would such a backdoor work?
I think it'd be more accurate if one says "if IV were chosen arbitrary and / or unknown, then backdoors might be possible". 

From what I know, the SHA256 IVs are chosing from the fraction parts of well known irrational numbers (though I'm not sure it's sqrt(2) or pi or something else). This way, the only being(s) capable of crafting a backdoor, would be the supreme being(s) beyond our Universe. 

Similarly, in RFC 3526, modulus is chosing using pi to avoid backdoors. As a side note, sadly, small modulus are vulnerable to mass pre-computation attacks.