Different Risk-profile - Shareholders and Management
Shareholders will generally prefer high-risk-high return investments while they are diversified that is they have many investments and the collapse of one firm may have irrelevant effects upon their overall wealth.
Managers on the other hand, will prefer low risk-low return investment because they have a personal fear of losing their jobs whether the projects collapse. Like human capital is not diversifiable. This dissimilarity in risk profile is a cause of conflict of interest while shareholders will forego some profits whenever low-return projects are undertaken.