Saw the video. I am guessing heart failure. He always put a lot of effort into his jumps that maybe his heart just gave out.
Just an addition this is what I mean by putting ''effort'' into his jumping...like over the standard.
Dailymotion - La Baule 2011 CSIO 5* Grand Prix 1 Jumping 15/05/2011 - une vidéo Sports et Extrême