For those who wish to see what happened in first-person and in full detail, as it aired live on tv:
(Caution: it IS graphic - this is not video of Hickstead's round but rather of his going down)
I've never seen a horse go down like that, but many with such experience are saying it looks like the equine version of a heart attack.
I still can't believe it happened; I saw him last in September at Spruce Meadows, competing, alive and healthy.
I am REALLY anxious to see how his foals go; Eric bought a foal by Hickstead recently: http://www.woodlawnfarms.com/fortknox.htm.
Edited to add:
Here is the full 34min CBC press conference with Eric Lamaze present. COD was determined upon necropsy to be acute aortic rupture.
Spruce Meadows has made a video tribute to Hickstead.
A wonderful article, Hickstead: The Little Stallion who Became Canada's Champion.
CTV broadcast, including Hickstead and Eric's last round at Verona, Italy, before the stallion collapsed (does not show the collapse).