Hand-raising experiences
I see this behavior a lot at conventions and sometimes even at meetings at work. I used to do it all the time myself, but I'm vaguely trying not to these days:
You raise your hand to indicate to the moderator or the person running the meeting that you want to be called on. They either fail to call on anyone, or call on someone else. You're now stuck sitting there with your hand in the air, and it may be several minutes before they're going to call on anyone again. So you have a choice: you can either keep your hand up and look like a dork, holding your hand in the air for the next few minutes while other stuff is going on, or you can lower your hand, thereby admitting that you had your hand up at the wrong time.
The solution? Drop your hand just as far as your head, and brush back your hair, or rub your neck, or stretch and yawn a little bit, and at the end of the gesture, drop your hand instead of putting it back up. This way you can give the appearance of not going out of your way to put your hand down; no, you had a perfectly natural other gesture to perform, and at the end of it you just didn't put your hand back up.
(Also, if while you're making the gesture it becomes possible to be called on again, you can get your hand in the air quicker than you could've if you lowered it.)
There is not, of course, anything wrong with this behavior. But I find it interesting that it's so widespread. (At least among fans and computer-company employees; do non-geeks do this? Do students in class do it? I don't remember, though I'm pretty sure I did it in high school and college.) Is it really so embarrassing to just lower your hand? I'm trying to train myself to do that, because even though there's nothing wrong with the head-brushing gesture, I do it as a nervous habit, and I'd rather not have nervous habits. But it's a hard habit to break.
I also think it's interesting that the gesture thing seems to be largely or entirely unconscious. It took me years to really notice that I was doing it.