A fork in software engineering allows people to take open source software and work on it in their own environment. This could be for a variety of reasons such as they want to experiment with changes without affecting the original project or resolve a bug that has been found and then submit a "pull request" to the original project to resolve the issue.
In short, its nothing to worry about and is common practice. On our public repositories we use open software licenses which allow for this to happen.
More information about forking can be found here.