Why do flies circle around ceiling light fittings? Light on or off the same result. Sometimes as many as 3, 4 in a holding pattern like planes do before landing. (Ah! the mysteries of life).
They're using the light fitting as a reference point to circle around, in the hope that other flies come along and do the same. The further hope is that there may be a fly of the opposite sex amongst any new arrivals. The light fitting is in effect a 'chat-up station'.
It's for the same reason that small groups of midges will form in the same spot in a garden each evening.