If pie trays are stacked on top of a Van de Graaff they will fly into the air one after another. Why does this happen?

The rubber belt inside the Van de Graff generator runs between two rollers made of different materials, causing electrons to transfer from the rollers onto the rubber belt, that are later collected at brushes acting as a sink for these charges. These brushes make contact with the dome of the Van de Graff and electrons get transferred to the dome of the Van de Graaf generator. IF pie trays are stacked on top of the dome, some of the charge gets transffered to the aluminium pie trays. As both the pie tray and the dome are now negatively charged, they repel each other and the top most pie tray flies off into the air. The process then repeats until all the pie trays fly into the air one after another.