I would say it has something to do with the amount of baggage being expected, as the forward holds are still being used exclusively (where possible) for a quicker baggage off/on load.
Travelled on 2 flights yesterday and this is my observation. First sector, 60% load but very little checked baggage, majority of PAX sat towards the front of the aircraft, a few passengers asked to move to the back before departure. But rows 22-33 were empty.
Second sector, lot of checked baggage about 75% load and more evenly spread throughout the cabin, overheard the dispatcher tell the crew there must be 18 people in the rear section (last 5 rows).