found bunny dead in cage

Question
I work the midnight shift and came home to find my bunny dead.  When I left for work he was fine and ate good.  No signs of any thing wrong.  I made sure he had fresh hay, water and fresh vegetables before I left for work as I always do.  When I came home in the morning, bunny was stretched out in his cage dead.  I was shock.  I just don't understand what happened.  I took him to the vet for an autopsy and the vet said that there was nothing wrong and was just as shocked as I was. I thought that maybe he chocked on something, because he likes to shred newspaper, but the autopsy didn't show any signs of choking.  She examined the kidneys and GI tract and didn't show any thing wrong.  She said he looked perfectly healthy inside.  So I'm confused at what the cause of death could be.  I have 3 cats, but the cats don't go near the bunny.  And the bunny was in his cage at the time of death, but I was wondering if he got scared in some way by the cats.  When I leave him run around the house he tries to chase the cats but they run from him.  So I figured he was use to having them around and new them, so maybe he couldn't have gotten scared by them.  I don't know why this happened, but I'm trying to get an answer.  I keep trying to figure it out and I just don't understand.  

Answer
Was the vet you took him to knowledgeable in rabbits? There are a lot of things that can kill a rabbit without very obvious signs if the vet does not know what to look for. It is possible that shock/fright killed him, but if he was used to the cats around, it is highly unlikely that they were the reason.

There are a lot of diseases that can kill swiftly and leave little trace, so a vet would have to know what s/he was looking for.

Was it hot? Rabbits can succumb to heat pretty easily. Did the vet mention anything about the digestive tract looking normal? Digestive problems are probably the most common cause of death in rabbits. Any blockage (hair, a ball of dead parasitic worms, anything non-digestable that he ingested, etc.) can cause death.

There could have been a cardiac problem, a liver problem, kidney problems, etc. Finding those all depends on the expertise of the vet.