It means usually overdose or suicide but could mean anything the family just doesnt want the details in the obituary. I have a brother who dies at 27, and a cousin who died at 27.. my brother of an overdose, his obituary said suddenly... cousin of complications due to surgery still said died suddenly. If it were a car accident or anything like that their would be a seperate report for that or a news article..
Ususally anything unexpected. A very close family friend of ours passed away last week in a car accident, his obituary said died suddenly, but also my mom passed away 4 years ago from cancer, but no one knew she had it, so her's also read died suddenly.