Change within a project can occur from a variety of sources. PMI specifically lists the following:
Errors and omissions
These events could include any event external to the project that causes a change in the direction or scope of the project. Examples include changes in corporate strategy, changes in project scope, and any new law that requires a product to conform to a specification not targeted in the project. It could also include something as dramatic as the World Trade Center tragedy.
Errors committed and omissions of critical items are the most likely reasons for change in the real world, although PMI strongly encourages excellent ...