SHA-2 was designed by the NSA. Nobody is saying there is a backdoor.

I think it's established that NSA backdoors things. It doesn't mean they backdoor everything. But scrutiny is merited for each new thing NSA endorses and we have to wonder and ask why, and it's enough that if we can't explain why something is a certain way and not another, it's not improbable that we should be cautious of that and call it out. This is how they've operated for decades.