The most common reason is that the inmate did something to lose the privilege. Work release is not a right, it is an earned status granted to inmates who have demonstrated they can be trusted with limited freedom in the community. When that trust is broken, the program ends and the inmate goes back to a standard facility.
The specific violations that trigger removal vary but typically include failing to report to work, coming back to the facility late, testing positive for drugs or alcohol, making unauthorized stops during transit, contact with prohibited individuals, or any new criminal conduct while in the community. Work release carries strict conditions, and violations are treated seriously because the privilege involves the inmate moving freely outside the institution during work hours.
Less commonly, an administrative change on the facility end, such as the program losing its contract or the inmate's sentence structure changing, can also result in a transfer, though those situations are less common than a conduct-based removal.
If this happened to someone close to you and you do not know what caused it, the inmate will know the reason. Their case manager or counselor can also provide clarification once the transfer is processed.