it’s because you’re scared of it. 2nd post has it – you’re tensing up in the approach and also in the air.
approach the take off with your energy pre-loaded, by that i mean in the ‘attack’ or ‘ready’ position: knees and shoulders bent & body weight generally low. as you go up the kicker extend your limbs and push up and out to lift the bike upwards – this will propel you into the air. as you feel the bike reach the crest of the trajectory you should aim to bring the bike up further still using your hands and feet. if you do this correctly it will give you that little more time to correct your mid-air balance, in fact, by doing this that is what you are doing. as you begin to fall again extend your arms and legs and then absorb your body weight with your knees and elbows once your wheels touch down.
or something like that. sounds more difficult than it is, you just need to practice.