That is scary, you did better than me, it has happened to me twice on my own horse, but not prepared for it. Never expected it. My mare wanted to get back to the barn to her buddy. I was not able to hold on and took a pretty hard fall. Had a sore back for days.
I am very cautious now and look for signs, like everyone mentioned here. And definetely one rein stop, you should practice though because it is not as easy as you may think. Especially of the horse is not used to having his head/neck bent all the time. Anyhow, good luck for the future.