I see this a lot pre-flop in hold’em. Players do a one-handed shuffle of their cards when they are planning on folding. I think it’s probably just a nervous release of energy—something to do while you wait to fold. Think of it this way; a player with a good hand usually doesn’t want to draw attention to himself at all, even in such a small way.
I also have noticed the tendency in myself, even though I’ve consciously tried to stop doing it.