Why would they move my inmate off work release to the main facility?

It could be for a number of reasons. If you call the facility, they may or may not tell you why. Work release is reserved for inmates with the best record while incarcerated. Work release is a privilege as the inmates are working outside the complex with a minor supervision and a high level of trust. If something happened that would remove that status, it might be why they've taken him off work release. Or it could be something else altogether.