Pre-boards and the priority boarding are two different things, in the order outlined above.

It is the interpretation of who gains pre boarding that appears to be the problem in this case. Anyone who requires assistance should and normally is dealt with first, followed by family's with small children. It doesn't appear to be defined clearly, but should only be extended to those travelling with infants in my eyes. If not, on family heavy routes you find that half the aircraft will pre-board as what seems to have happened here. Imagine the stampede on the MCO route during the holiday season for instance if it is not policed correctly!!