Simple people put an NTR tag the person doing it must be a horse-cocked black dude. It is playing the cuckold stereotype, because for some reason NTR has changed to cuckold.
There is a lot more to it than that.
The hung black guy plays on the humiliation of the tiny cocked white man, it's part of cuckold fetish and also plays a part in NTR. No woman is going to be stolen away by a weak willed, tiny cocked rival. it just wouldn't work.
The other part of it is aims more at women because a lot of women have a fetish for big black cock.
I have friends that go for that very thing, they also enjoy things like this.
A lot of the fem forums I pop into every now and then have sections dedicated to cuck fetish and how they want to fuck hung black guys infront of their partners.
Most of them will never do it but it's fantasy. Some guys also want to see their partners with other men whether it's just to make them happy or it's a kink they have themselves.
The thing with cucking and NTR is no woman's going to cheat on her partner with a shit lay so it needs to be someone that puts their partner to shame.
Even then we're only scratching the surface, there is a lot to it.