Thats actually quite common. I believe that subconsciously the mind tells the body to protect itself, and since falling is a sign of danger the body reacts by locking up. practice and calm nerves can help you train yourself to adjust to it.
yuh..... two letters and you can't spell it right?! jk I don't care. lol It's because of the shock of falling. specially from a high place. It's just fear. The minds natural reaction to certain things really. Surprise of suddenly being that airborne all of a sudden. etc. You have to really train your mind to overcome these natural instincts of fear. Like overcoming the fear of the dark or something like that
Because your protective shell has been compromised. You are facing death as your inner psyche sees it in an un-known state of departure. Scary because you have no inner peace. Think! and please comment. There's more