as far as i know there is nothing to suggest how he became a skeleton in "the nightmare before christmas" i think tim burton just wanted to create another unique film, that one revolving around dead characters like jack and zero and the rest of the cast like sally who as a stitched up doll, obviously isnt "living" like a human would. i beleive all of the characters are kind of "dead", with jack being a skeleton and zero being a ghost dog, ect
After being shot by the U.S. military, everyone in Halloween Town believes Jack was "blown to smithereens" and he does appear dead until Zero places his jaw in the right place. He also mentions in the subsequent song that remaining in a cave for million years would turn him to dust, hinting that he may not be immortal and will one day wither away.
Not to be mean but like obviously they have had to be a human to be dead, in the movie ,in "Whats this" he metions something about when he was a boy
he was already dead in the first place and thats as close to dead he has ever gotten.
You can ask Tim Burton, he created Jack Skellington.
No i mean like how did he become a skellington ?