@Qelix It is as he stated. Racism. Because she is a demi-human which the demons cannot stand, it does not matter if she is in charge if you do not respect them.
Demi-humans and demons don't like each other, so throw a third race in for both sides to mutually hate thus establishing common ground. If it works in the real world(sadly, it does) should work here as well.
@jonsmth, @Anarchyant: Refusing to follow the orders of your commanding officer in situation of crisis, because of racism? Well, I guess it doesn't really matter which other stupid they may pick, but it does make a difference if they refuse the orders or not. Depending on the army they serve in it can be literary a matter of life and death for the subordinates.