I wonder what the Captain was doing in the rear galley...
Thread drift but to answer the question: No reason why he/she shouldn't be in the rear galley in the right circumstances -e.g a long flight where there might be a relief pilot and therefore each pilot gets some "time away from task" .
I certainly sometimes head there on the 777 if if I'm on a break, good for the circulation and also to see how it's going with the rear galley crew.