I don't know if it was for the same reason, but last night they had tower (who was very busy anyway) controlling all the ground traffic on the south side as well. It was foggy for sure.
Combined frequencies...perhaps so one controller knows all the given taxi instructions on and near runways in the low visibility? None of parallel taxiways on that side are particularly far from the 25's, and maybe to get rid the communication time between ground and tower controllers up there they kept everyone on the same radio to be safe.
Pure speculation here, btw.