I've never been to one of Apple's events, but as a viewer of their videos some of those applause breaks have me convinced there are giant "Applause Now" signs. I imagine them being on more than they are off by how over the top their dialog is written. Or they fill the air in the room with some sort of gas that makes the attendees very susceptible to the RDF.

Those are much more fun explanations than just realizing they probably have plants to start the cheering in a room full of fanboys