MRB convinced XZ's dad with words, can't recall the justification used. XZ's dad believed him because MRB was a friend and was respected. The rest of wulin trusted the dad, and the rest is historyYes, it was vague. I can't remember the exact wording in the third edition, but I don't think the novel explained how Murong Bo justified his false accusation. I would, however, assume that the novel simply omitted the detail (it was only mentioned briefly, as 'ancient history'), so we just don't know the circumstances. One would think it's inconceivable wulin wouldn't have taken serious convincing to risk losing lives and causing a major political disaster.
This and the MRF case makes me look at wulin as a mass of mindless zombies with just a few brains between them.
Which is why TLBB is my favourite of JY's books and I cannot stand SPW and ROCH. I prefer GL who generally have better plots and schemes.That's Jinyongian plotting for you. Not his strong point. At least, in this respect, TLBB isn't as bad as the 'Trilogy' novels!