There are a lot of great rabbit books and online sources out there that will talk in depth about rabbit diseases and illnesses. The obvious causes are age and respiratory problems, but any number of things can go wrong with a bunny. If you're truly curious look up official sources rather than asking people's opinions because they can give information above and beyond what most typical rabbit owners are aware of - you might just find some "symptom" that your rabbit might have been displaying (that you might have not registered) that might indicate what illness had been involved in his/her death. My friends visually healthy rabbit died abruptly one day from being fed the wrong thing that caused the rabbit's stools to get stuck. Unfortunately this problem wasn't noticed by the owner in time and the rabbit passed on. The problem with the food came to light a few days later and the word was passed on to her. (but don't worry it wasn't a manufacturer of rabbit food it was a provate source for her. Needless to say she switched to manufactured rabbit food)
