What are the reasons that a cause of death may not be listed in an obituary?

I just found out that a friend I’ve had since I was very young passed away recently. It was one of those situations where I could kick myself because I hadn’t heard from him in a few weeks, and kept saying that I should write an email or send a message, see how things were going, and then never did.
Today is his birthday and I went to his facebook page to post him a message and saw all of the posts from other friends and family about his passing. I looked up the obituary and I missed the funeral, which was yesterday, and all that it said about his death was that he was found at home, and had died unexpectedly. It isn’t even 5am, so I can’t call anyone to ask if they know what happened right this second.

What could that possibly mean? Are the possibilities limitless, or do they typically use a vague description like that for special circumstances?