Complex sentence: The jockeys in the race and the track stewards reported that they had not witnessed any incident to have caused such a fall; it is generally believed his diabetic condition may have resulted in his suffering a dizzy spell or fainting.
Split into simpler sentences: The jockeys in the race and the track stewards reported that they had not witnessed any incident during the race to have caused such a fall. Most observers think his diabetic condition may have resulted in his suffering a dizzy spell or fainting.