Pass on my sympathies, we all dread the loss of our family, even when you know it is coming you can't realy be ready, at least they were there & did not just find him dead.
from the description it sounds almost like he heard someone call him to go doesn't it?
A bad experience for the humans but better for him to go like that, i am sure we all agree on that,
a friends 4y old dropped down dead in the field still with grass in his mouth due to a brain haemorage,
one of the un-known maybe's of any horse which may have had a bad patch in its life is *aneurisms which can sit there and blow out years later, this was thought to be the route cause of my friends horses death.
*aneurism is when a blood vessel expands and then leaves a weakened blood vessel wall, rather like a once blown up balloon now gone down, although they can be present from many causes in any horse (or other animal inc us) they are often cause by a build up of red worm in the system as this sometimes causes a clogged up bloodvessel and behind the clog the bloodvessel swells.
ANOTHER GOOD REASON TO KEEP ON TOP OF WORMING GUYS!