It's orchestrated, of course, like many things in public life (not just politics). When we see someone nodding, our subconscious sends us a positive message. This increases our own sense of positivity, which we associate with whatever is being said. That's the theory, anyway. A lot of "ordinary" people use the nodding technique too when they're telling us something. We can't all have an entourage to do it for us, so nodding while they're speaking increases the chance that we'll be convinced that they're right. A huge number of young women seem to be doing it lately, so I imagine one of the glossy mags or online self-help blogs has run an article on "how to make friends and influence people"....
I'm nodding while I type this, btw, so I hope you're all now in total agreement with me. 