You are ahead of the motion. In the picture you've thrown your body forward, letting your leg slide back behind you, instead of letting the horse jump up to you. You are literally jumping out of the saddle. Ideally you want to be over the middle of the saddle, with your legs lined up with the girth right under you. It also looks like you might be looking down, although it might just be you blinking.
I use to do that all the time; my trainer would always yell at me to treat the jump like a speed bump.