When a sentenced inmate serving time at a state prison gets pulled back to a county jail, there is almost always a court-related reason behind it. The Georgia Department of Corrections does not move inmates out of their designated facility without cause, and a transfer back to Fayette County specifically points to something happening in that county's court system.
There are two common explanations. The first is a new charge. If something has come to light, whether a crime that predates the current sentence or new conduct that has been investigated, the county can file new charges and request that the inmate be produced for arraignment, hearings, or trial. A new charge does not require the original sentence to be complete first. The county issues a writ, the state transfers custody temporarily, and the inmate goes through the court process on the new matter.
The second possibility is that he is needed as a material witness in another case. Prosecutors sometimes need testimony from incarcerated individuals in cases involving other defendants. If he has knowledge relevant to a case being tried in Fayette County, the court can compel his presence through a writ of habeas corpus ad testificandum, which is a legal order requiring a prisoner to be transported for testimony.
Either way, the move is temporary. He will be held at Fayette County Jail for the duration of whatever court proceeding requires his presence and then returned to state custody afterward.
His attorney or the Fayette County Clerk of Court can tell you which case is driving the transfer and what the hearing schedule looks like.