It looks like it is square or rectangular in shape and rotated a few degrees clockwise. Is that the shape of your lens hood or is it one of the standard "Petal" hoods? Shooting out of a window or something?
The lens hood was partially rotated, not mounted right. If it were seated the way it is supposed to be those dark corners would have been rotated just enough to not be seen. Give it a try sometime on purpose and you will find a similar result. the petals are designed to a pretty tight tolerance to use all the available lens at an ultra wide angle.