on the drops your thumb is curled around the bar with your hand having no where to go but up or down.
On a flat bar, again the bar in between your thumb / fingers and in your fist. Your hands can't go forward without ripping your thumbs off.
So here is the Pop quiz: What is your thumb wrapped around on the hood to keep your hand from sliding up and off the hood?
Edit: someone might be blurring the line between group ride, and club ride. lol