In Episode 30 of the latest Jojo series, JoJo no Kimyou na Bouken Part 5: Ougon no Kaze, Giorno notices that Bucciarati became an undead and Bucciarati hypothesizes that his strong will was probably a factor at play. That, however, does not add up because lots of people who have died including Abbacchio and the previous Passione Capos had very strong will.
My question comes in 2 parts. Firstly, apart from a strong will, what other factor(s) cause a person to become an undead after they die? Secondly, why are the Stands of the undead not affected by harm done to their physical bodies?