So sorry for your loss. Bunns are way too good at hiding problems from us. We've rescued 50 and had the same thing happen several times--fine in the morning, or so it seemed and then gone later that same day. All we can do is love and take care of them the best we can. Have had several make it to 15 and 16 and Bambi was just a few weeks away from 19--also lost a few before they hit 1--just how it is.
Lots of love from me and Pumpkin. I know how it feels to lose a rabbit. Sometimes it's just out of your control. The important thing is that you gave him an amazing life to live and he had such a loving owner. I'm sure he died happy considering there was hay in his mouth. He's doing binkies in rabbit heaven.