What happens is you're so relaxed your esophagus opens and doesn't reject the vomit as it normally would, so if no one's there to turn you on your side and clear air passages you choke on it.
This just happened last week to a man I knew. He was in his late twenties and not, I think, doing more drugs than many his age who feel a little lost in life. It's as nearly a death by chance as someone in high spirits not thinking to look before stepping into a street. That momentary lapse in judgement has robbed him of decades of life.
The older you get, the more shocking such fragility becomes to you.