I read his book "Still Me" and he said that he was trying out lots of dangerous sports, the last one being riding and jumping. On another thread, someone said that his trainers were advising him against jumping the courses he was riding, telling him he wasn't ready. Any way about it, he took a big risk and lost.
As someone who has ridden my entire life, I agree with your comments about Reeve's riding ability. I saw the footage of the accident, and he made a common novice mistake called 'jumping up the horse's neck'. Instead of letting the horse's momentum lift him out of the saddle over the jump he anticipated the jump and got in position before the horse was ready. He was a big, heavy guy and the horse decided it couldn't get over the jump with its rider's weight off balance and slammed on the brakes.
Reeve was incredibly unlucky to have this accident result in a broken neck, however. I've had the same thing happen to me. I've fallen off in every way imaginable and have always walked away. If you ride horses you are going to part company every so often - that's just part of the sport.