I would never dream of asking how the deceased died. There was some speculation, and then the real cause of death did make it's rounds, and I eventually learned what happened.
I'm not sure what Ms. Manners' official response is, but it seems to me that the only appropriate time to ask how someone (someone who is not in your family) died is immediately when you find out.
"Billy-bob died? Oh my god - what happened?"
IMO, If you miss that window - or if the person who told you doesn't know - you do not get to ask again. You will either find out later, or you'll have to make peace with not knowing.
Of course, if it's family or a person you've numbered among close friends - like, someone who regularly helps you clean up after parties, someone who you have bailed out of jail at least twice or vice versa; someone for whom you have at least two stars next to your name in their contact list; or someone for whom you are listed in their Close Friends Groups on Google+ or Facebook - then you probably have more leeway, and wouldn't hesitate to ask anyway.
IOW: If you have to ask what the etiquette is, you probably shouldn't ask how they died - the exception being the immediate response to the news.