As Cherie mentioned, going fast backward is dangerous and can get you and your horse hurt. Hopping off when it is happening can actually promote the behavior, but in certain cases that's all you can do. I know from experience on that.
Rule out all the possible physical causes you can, such as a bit pinching, too tight or twisted chin strap, abscess or injury in the mouth, sore back, etc., before deciding whether it is a training issue.
If your horse is a Quarter Horse, google "hyperkalemic periodic paralysis" and see whether this might be a possibility. I have had two QHs that suffered from this. They would occasionally just seize up and stop. If I did anything other than get off when that happened, they would literally explode, running backwards uncontrollably in a panic until they fell over backwards. Nearly got me a couple times. One nearly killed my dad by backing off a steep mountain trail. I thought it was simply misbehavior by the horse until I began to research it a little. Some studies indicate it affects 1 in 50 Quarter Horses. I have never heard of it affecting any other breed, but I suppose it could. Both of my horses (one the foal of the other) had excellent "Three-Bar" QH breeding. Some say it is more common in the "Bar" lines than others. It seems to be a malady that causes intense charley-horses or muscle cramps, and it appears to be hereditary. It affected our horses in their older age, upwards of 16, but never a hint in their earlier years.
So now I pretty much stick with mixed-breed "grade" horses that just look and feel right to me and stay away from registered animals altogether.