This clip is what is known as an "out-take".
Many movies end with these while the credits are rolling - they're hilarious
(perhaps the most famous for her flubs is Carol Burnett - the entire cast was often reduced to uncontrollable giggling, and that was in the day of live television ... but, I digress).
Bush was being recorded, and somebody off-camera pointed out that his hair wasn't straight
(see the little forelock? He attempted to correct it, unsuccessfully).
Going off on a tangent - with an entire nation watching his every move, the pressure upon him is so great that armchair journalists would have a field day if his hair wasn't straight ... but, I digress.
Bush realised that they'd have to start over, so he had a little fun by flipping the bird.
He's human.
To sum up, I elected to add this summary on behalf of any users who might have wanted to add some intelligent commentary (the Yanks call it their right to 'free speech') but couldn't because this thread was locked.