This was explained on Essentials of Strategic Management written by Charles W. L. Hill, and Gareth R. Jones p.46.
The authors explained this on The Roots of Unethical Behavior that managers are still individuals, it means that a person's behavior still relies on personal ethics.
Another reason is that managers at times might not see the relationship between a business approach and the ethical space. They might believe that it is merely a business decision and they go beyond entering the ethical dimension.
And last is the organizational economic approach to have a goal which can push them to disregard their behavior.