Andy, that is the nature of motorcycles, they can be unpredictable at times and unfortunately when that happens, it sucks!
I rode for 28 years and I got it wrong and it took nearly 10 years to recovery from it. Watching Simoncelli go down was gut retching and sent a deep chill through me, for I know how that feels. It is a sad day indeed when we lose talented riders. Unfortunately, although infrequently, it will probably happen again. It is a very dangerous sport.