The instructor explaining what happened, as she believes it to be, isn't unprofessional. I would much rather have heard that, than *shrug* "I don't know."
Do not beat yourself up about it. If it *was* you, the horse will forgive you, and whatever was wrong will heal. Your instructor did see a flaw in your riding, whether or not it was the cause of that little ordeal, and will be focusing on it until you are more correct in that area.
My last day riding one semester in college, I was assigned a horse that I had ridden five or so times before. I knew him. He was a steady, lazy, plodder. A good horse.
I tacked him up properly, went through the routine, got on him, adjusted my saddle, walked him off, and as we rounded the corner, that sucker took off bucking. Everything was in slow motion. I was thinking "What do I do? Um... One rein stop?" That turned him bucking across the arena. In the middle I heard "Get your legs out of him!" from my coach. I was not coming off in my mind, so I grabbed him, which more than likely prolonged the bucking. I aimed him for the wall. Luckily, he stopped completely at the wall and I jumped off him. My coach though it might have been the navajo thing that caused it. I doubt it, but the explanation was good enough for me.