It's pretty straightforward. Weak manchildren know they can't resist if challenged by dominant men, so they make this face to show they're harmless. It's the primate-brain way of saying "Please don't hurt me, look how happy and nonthreatening I am." They're roleplaying as children to avoid having their asses kicked, basically, and the funniest part is they're not even aware they're doing it, it's entirely instinctual.