Give a Dog a Bone
It had started as an act of compassion.
Brook had been working himself to the bone (Skull Joke, Yohohoho!) to help save the Minks along with every other able-bodied person when one of the Canine Minks had started acting up. The poor thing was delirious from the poison and frightened, so he was biting at anyone who came near, including those trying to treat him. Brook's first thought had been to use Nemuriuta: Flanc to put the Mink to sleep, but he quickly decided against it. While Brook had improved his ability to target specific people with his music, the fact that most everyone here was tired and stressed left them too open to his music's hypnotic abilities, especially the power of a lullaby. So to save the others from being bitten, Brook shoved his own forearm into the Canine Mink's mouth. He wouldn't bleed, didn't have flesh to tear nor nerves to set on fire with pain (though it was a little uncomfortable), his bones were strong and healthy, the Mink, not so much, and a few easily healed dents in his arm bones was a small sacrifice to pay to save a life in Brook's book.
While it had surprised those around him, they couldn't argue the effectiveness of the move when the Mink settled down, slobbering as he chewed Brook's forearm.
Later, word of this "generous" act got out to other Canine Minks, who took it as an invitation from their delectably boney savior to participate in a tasty new form of minkship.
As for Brook, he came to the unfortunate realization that if you give a dog a bone, it will come back for more, and brings its friends too.
Autor's Note
As I stated back in Some Assembly Required Brook let the Minks chew on him, so I decided to do something on how that got started because as touchy-feely as the Minks are, with how rough the dogs were with Brook (especially compared to the others), I think they needed some form of "permission" first.