These aircraft are too stable and anhedral is added to make them more unstable and maneuverable.
It takes very little aileron movement to make them roll and once they start to roll they want to keep going, on the Harrier anyway. Imagine once one side of the wing is horizontal it is creating the most lift it can make and the other side is
down some 20 degrees or so it is not lifting as much so the roll wants to continue.