It is know as the opposition effect.
It is not something that is unique to cameras, our eyes see it as well as it's just physics. It happens all the time but is only noticeable in certain situations - mostly on uniform crops due to the texture of the subject and acute angle of the crops causing reflected light to go off axis from our view point but is also viewable on other surfaces.
As light travels in straight lines (for the purposes of this phenomonen anyway) what we see in the world is made up of reflected light. (We Don't actually see any objects at all, only the light reflected from them)
In this scenario, when we view the surface from above with the sun directly behind and at the same angle most of the light is reflected at angles off the surface which means what you see is very slightly darker. The bright spot is the light reflected in exactly the same plane as your viewpoint and angle and therefore very little of the light is lost in shadow/reflection. This then appears as a slightly brighter area from our viewpoint.
It's not JUST crops though, it happens on nearly all surfaces.