She has a lot of experience breeding, she had at that time a splendid Thoroughbred stallion who had been a great show jumper in his day, with a lovely sweet temperament (she could keep him in a barn full of mares, no problem, he never got pushy or possessive, and anybody could ride him.) She bred him to one of her warmblood mares. The colt was always handled gently from birth and never forced or trained roughly.
He was just a mean S.O.B. and was so from about the time he turned 2. Maybe he was psycho.
Yikes, a warmblood crossed with a TB is just too big to afford to be mean.