This image was recently posted on Dressage Academy's Facebook page and I'm afraid I find it hilarious. There's something very amusing about seeing someone having a fright - in this case that someone being a 800kg horse.
A fright involves what's known as the 'startle pattern'.
In human beings the startle pattern begins in the neck and passes through the entire body in less than half a second. The neck retracts violently; the back tenses; the arms and legs are drawn up; the breath is drawn in with a sudden gasp, sometimes a shriek and we potentially leave the ground.
From this image I would assume the pattern is similar in horses - probably in all vertebrates.
It is a reflex response to fear, in other words it's not something we have any conscious control over. We can't help jumping if we have a fright.
I've seen horses worry over the smallest things - from puddles to woolly hats. If you try to force a horse to overcome these fears you will end up with a tense, neurotic animal. Gentle coaxing and lots of patience is what they need.
Let's save horse jumping for the arena:)