Sorry to hear. I'm guessing it is probably a really uncommon and sudden thing that is also rather complex and not well understood. Perhaps the head moved a certain way and it dislocated the spine enough to squish vital nerves that triggered something else like the heart to stop, but then it popped back, or something weird like that. It would have to be something that "reverts" leaving no evidence. I'm sure the doctors must have some speculations, but probably not strong enough to make an official statement.
Did they even mention any possibilities, or ask any leading questions, such as if he had allergies or something?