The Stampede says in a release the horse suffered from a weak blood vessel in the heart that burst, causing blood to fill the animal's abdomen.
Chief veterinarian Greg Evans says the condition is undetectable in animals and could have ruptured at any time during exercise.
That horse's collapse caused a crash that injured two others so badly they had to be euthanized on the track.
A fourth horse needed surgery and is expected to survive.
The Vancouver Humane Society wants the chuckwagon races to be shut down while an investigation is conducted.