This sort of shot is difficult, however, would a little more of the rider's face add more to the composition? I can see from the light why you have chosen your stance, was there a little more space to the right to stand for the shot? (Not only that I know that some fences are taken slighty different by each rider so you can never be in the right place ALL of the time )
Yes it's always difficult -like you said- I never know were the rider will jump until the last split second, and not only that, some riders has a style that often makes them disappear behind the horse. In this case the rider lost the rythm between the jumps but the horse amazingly saved the situation!