What I am seeing is the beginning of an automatic release. It's not too bad. I think some other people are worried that you are using the reins to "pull" you over the jump. While there may be a bit more pressure on the reins than I would like to see, it really doesn't appear to be that bad. While I agree that your shoulders could be a little too rounded, I would rather see that than the stiff postured back I see on many hunter riders. Yup, I do event and jumpers. I believe in a SOFT back to allow even softer hands and seat. The stiff backs, and resulting stiff hands/seat, are why crest releases and seats unable to sit the canter these days. The reliance on the crest release keeps you from developing the seat/position independant of the "prop".
Whew! Flame away.....
As you can see from my photo, my equitation isn't too terrible (except when I am galloping jumps on XC). I do NOT have the arched back so loved in the hunter ring. If I did, I would not be able to ride these jumps the way I want to....balanced and with the horse.
Try to bring your shoulder blades back, but keep the soft back. Come event!!