"One of whom" or "One of them"?

I come across the sentence below,

"The show's creators- one of WHOM" is himself a Rastafarian- insist that no offence was intended"

I wonder why it's "one of whom" but not "one of them"

I come across the sentence below,

"The show's creators- one of THEM" is himself a Rastafarian- insist that no offence was intended"

Please tell me the grammatical rule behind the usage of "whom"

Thanks a lot