I agree, all sorts of weenies do this. Usually if they are a young kid and obviously not your usual casino fish, it's nice because you know they have a big hand.

It's a form of overacting. And usually they are not even that good at it (especially if it's so noticable that it looks like they want to muck, it's smarter to at least try to be more subtle about it so you might think that you picked up something that they are unaware of)