I think, as the below answer states, you mean external factors (that have nothing to do with the company itself) that may hinder the progress of the project. Such external factors may include:
- New Regulations: New regulations may wreak havoc on a project. For example, when the ban of offshore drilling was imposed, all the projects related to that industry were killed or postponed.
- Instability in the supply chain: Sometimes vendors do not deliver the material on time, and this affects the project
- Recession: There will be a very high chance, in case of a recession, that the project (once done) will not return the predicted ROI.
- Country instability: for example, because of the turmoil in the Middle East, a lot of projects are being put on hold
Other external factors include: Unions, natural disasters, inflation, competition, etc...