Nothing. I can do it both ways, but middle finger is always more fiddling to get the knife in the hand just right, and then more grip adjustment afterwards, than if I had just index or thumb flicked it open. I, too, once tried to understand the reason for it, and well, ultimately came up empty. But if it's fun for people, great, just don't cut yourself or someone else.
What I really want to know is how Mike Janich is so good a pinky flicking a Delica of all things. Or those guys who can one hand open and close an Endura by pinching the scales with their thumb and pinky/ring/middle fingers while actuating the lock bar with their index finger. Open and closed fast and easy just like PM2 or CBBL knife. That amazes me and makes me just a teeny bit jealous.