My understanding, based on personal observations and knowledge, is that at the appropriate point of a flight as part of their pre-landing checks, the flight crew would activate the 'seat belt' sign, the cabin crew would then make a PA to the effect of seats to the upright position, tray tables up, seat belts on etc. and do a walk-round or two checking that all pax have done as instructed before informing the flight crew, to the effect, "cabin secure" or as one Australian purser I knew would joke "set to jet", and unless the flight crew have received that feedback from the cabin crew then their pre-landing checks are not complete and they, as it seemed they did on this occasion, perform a go around.
Why no 'seat belt' sign came on, that is a mystery!