I started doing it when I realized that my fingers would twitch in live games when I was bluffing. Couldn't control the twitch, so as long as I keep shuffling, you can't see that I'm twitching. Now it's just habit. Decided to play two short live tourneys a few weeks ago without shuffling. Got through it, no twitching anymore, but damn it was like quitting smoking - I just wanted to shuffle the chips all the time.