The concept here that charge resides on the outer surface of a conductor. As the globe of the generator charges up, the charge will move to the topmost pie tin. It accumulates charge and since it is not fixed in place, like the upper half of the globe itself, it will be repelled by electrostatic forces and hence fly off. Just after "liftoff, it retains its charge and is repelled from the next pie tin that now becomes charged as the outermost. Once it is gone, the second tin becomes the uppermost, and receives the full charge. It too will fly off. And so on until all "birds have left the nest".

Now, you might argue that the pie tins do not represent a solid conductor. There are obviously air spaces between the individual tins, so each tin itself might be accumulating charge. But in reality there is some metal-to-metal contact between tins, so the charges will seek out the path that allows them to be farthest away from the like-charged globe of the Van de Graaf.

Click here for more Van de Graaf demonstrations.