J2 SOP is to unload the rear hold first, and load the front first. Also cabin crew are instructed that when the front has bene de-planed via Airbridge or only front steps to make sure there is people in the front of the cabin. Hence why the stop you for a short while to allow the next row or two to progress to the forward half. It doesn't make a lot of difference really, you'll reach the back of the queue for security before it gets to the front the same.
They're quite conservative and it can be annoying but it's safer just in case.