How heavy handed are you? Is he trying to snatch the reins from your hands by jerking his head down? I find that horses tend to do that with riders that either pull or are heavy handed. Or have been ridden by other people who do that and have little tolerance for even the slightest pull. I'm guess it's more then just stretching down. Usually when horses are trying to stretch their back they lean heavily on the reins rather then a quick yank down that might unseat you. Also most horses trying to throw their riders won't try to throw you using just their mouths, they will add a little kick or buck into it. Although, I say most.... we had a little school pony who would be cantering and out of no here slam on the brakes and throw her head down. Catapulted lots of riders that way. :) But the kids sure learned how to sit up at the canter!