The most common reason is a consecutive sentence. When a judge signs a Judgment and Commitment order that includes multiple sentences running one after another rather than simultaneously, the second sentence does not start until the first one is completely finished. If your person was wrapping up a federal or county bid at MCRP and has an unresolved state sentence sitting behind it, the state can pick him up the moment he is released from the first facility, sometimes before he even walks out the door.
This is more common than most people realize and is one of the more painful situations in the system. From the outside it looks like the person is about to come home, and then suddenly they are gone again in a different direction.
Whether he now has to serve his full base term on the state sentence depends entirely on what that Judgment and Commitment document says. If the state sentence carries its own good time provisions, he may be able to earn credits toward an earlier release date just as he would on any other sentence. If there are mandatory minimums attached to the state charge, those will set the floor regardless of good behavior.
The fastest way to get clarity is to get a copy of both sentencing documents, the one he just finished serving under and the state order he is now being held on. His attorney or a prison legal aid organization can help read those documents and calculate what the actual timeline looks like. Do not rely on what facility staff tells you informally. The paperwork is what controls.